Method and apparatus for performing communication in a wireless communication system

ABSTRACT

A method and apparatus for performing communication in a wireless communication system are provided. The method includes identifying a transmission mode configured for a serving cell by a Base Station (BS), by a User Equipment (UE), identifying an antenna configuration of the BS by the UE, determining the number of bits for a Rank Indication (RI) representing the number of layers based on the transmission mode and the antenna configuration, and generating an RI using the determined number of bits and transmitting the RI in transmission resources of the serving cell to the BS.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

The present application is a continuation of U.S. patent applicationSer. No. 14/792,356 filed Jul. 6, 2015 which is related to and claimsthe benefit of U.S. patent application Ser. No. 14/093,257 filed Nov.29, 2013, now U.S. Pat. No. 9,078,163, which is related to and claimsthe benefit under 35 U.S.C. §119(a) of a Korean patent application filedin the Korean Intellectual Property Office on Nov. 28, 2012 and assignedSerial No. 10-2012-0136411, a Korean patent application filed in theKorean Intellectual Property Office on Jan. 18, 2013 and assigned SerialNo. 10-2013-0005876, a Korean patent application filed in the KoreanIntellectual Property Office on Jan. 28, 2013 and assigned Serial No.10-2013-0009091, and a Korean patent application filed in the KoreanIntellectual Property Office on Apr. 5, 2013 and assigned Serial No.10-2013-0037375, the entire disclosure of any of which is incorporatedherein by reference.

The present application also incorporates herein by reference in itsentirety U.S. patent application Ser. No. 14/792,356, titled “METHOD ANDAPPARATUS FOR PERFORMING COMMUNICATION IN A WIRELESS COMMUNICATIONSYSTEM” filed on Jul. 6, 2015.

TECHNICAL FIELD

The present disclosure relates generally to a communication system, andmore particularly, to a method and apparatus for transmitting andreceiving data in a wireless communication system.

BACKGROUND

Mobile communication systems have been developed to guarantee mobilityof users and enable communication for the users. Due to the drasticdevelopment of technology, the mobile communication systems providehigh-speed data communication service as well as voice communicationservice.

A future-generation mobile communication system, 3rd GenerationPartnership Project Long Term Evolution (3GPP LTE) achieves high-speedpacket communication at a data rate of up to 100 Mbps higher than 3GPPdata rates. In addition, an LTE-Advanced (LTE-A) system is underdiscussion, which increases data rates by applying new techniques to theLTE communication system. Hereinbelow, the term “LTE” covers both thelegacy LTE system and the LTE-A system.

The LTE standard supports both duplexing modes, Frequency DivisionDuplexing (FDD) and Time Division Duplexing (TDD). In FDD, differentfrequency bands are used for an UpLink (UL) and a DownLink (DL), whereasin TDD, the same frequency band is used for a UL and a DL.

One of new techniques to be introduced to the LTE-A system is CarrierAggregation (CA). In CA, a User Equipment (UE) transmits and receivesdata in multiple carriers. Specifically, the UE transmits and receivesdata in a plurality of aggregated carriers (generally, carriers servicedby the same evolved Node B (eNB)). Data transmission and reception ofthe UE in aggregated carriers is equivalent to data transmission andreception of the UE through a plurality of cells. Therefore, thereexists a need for a technique that enables a UE to reliably transmit andreceive data to and from an eNB, when CA is applied to TDD cells havingdifferent frequency bands and different subframe patterns.

The above information is presented as background information only toassist with an understanding of the present disclosure. No determinationhas been made, and no assertion is made, as to whether any of the abovemight be applicable as prior art with regard to the present disclosure.

SUMMARY

To address the above-discussed deficiencies, it is a primary object toprovide a method and apparatus for transmitting and receiving controlsignals required for data communication in a wireless communicationsystem.

Another aspect of the present disclosure is to provide a method andapparatus for measuring non-serving frequencies in a wirelesscommunication system supporting a plurality of frequencies.

Another aspect of the present disclosure is to provide a method andapparatus for configuring a measurement gap during which an UE suspendsan uplink transmission and a downlink reception in a wirelesscommunication system supporting a plurality of frequencies.

Another aspect of the present disclosure is to provide a method andapparatus for reporting a Rank Indication (RI) for a Multiple InputMultiple Output (MIMO) operation.

Another aspect of the present disclosure is to provide a method andapparatus for determining the number of bits (or the bit width) of an RIrepresenting the number of layers for spatial multiplexing.

Another aspect of the present disclosure is to provide a method andapparatus for transmitting and receiving data to and from a Base Station(BS) in a Time Division Duplexing (TDD) terminal, when the carriers ofTDD cells having different subframe patterns are aggregated.

Another aspect of the present disclosure is to provide a method andapparatus for transmitting and receiving data in aggregated carriers ofTDD cells having different subframe configurations.

In accordance with an aspect of the present disclosure, there isprovided a communication method in a wireless communication system. Themethod includes identifying a transmission mode configured for a servingcell by a Base Station (BS), by a User Equipment (UE), identifying anantenna configuration of the BS by the UE, determining the number ofbits for a Rank Indication (RI) representing the number of layers basedon the transmission mode and the antenna configuration, and generatingan RI using the determined number of bits and transmitting the RI intransmission resources of the serving cell to the BS by the UE.

In accordance with another aspect of the present disclosure, there isprovided a communication method in a wireless communication system. Themethod includes identifying a transmission mode configured in a servingcell for a User Equipment (UE) by a Base Station (BS), identifying anantenna configuration of the BS, determining the number of bits for aRank Indication (RI) representing the number of layers for based on thetransmission mode and the antenna configuration, and decoding the RIusing the determined number of bits, upon receipt of the RI intransmission resources of the serving cell from the UE.

In accordance with another aspect of the present disclosure, there isprovided an apparatus of a UE for performing communication in a wirelesscommunication system. The apparatus of the UE includes a controllerconfigured to identify a transmission mode configured for a serving cellby a Base Station (BS), to identify an antenna configuration of the BS,and to determine the number of bits for a Rank Indication (RI)representing the number of layers based on the transmission mode and theantenna configuration, and a transmitter configured to generate an RIusing the determined number of bits and to transmit the RI intransmission resources of the serving cell to the BS by the UE.

In accordance with another aspect of the present disclosure, there isprovided an apparatus of a BS for performing communication in a wirelesscommunication system. The apparatus of the BS includes a controllerconfigured to identify a transmission mode configured in a serving cellfor a User Equipment (UE) by the BS, to identify an antennaconfiguration of the BS, and to determine the number of bits for a RankIndication (RI) representing the number of layers based on thetransmission mode and the antenna configuration, and a receiverconfigured to decode the RI using the determined number of bits, uponreceipt of the RI in transmission resources of the serving cell from theUE.

Other aspects, advantages, and salient features of the disclosure willbecome apparent to those skilled in the art from the following detaileddescription, which, taken in conjunction with the annexed drawings,discloses exemplary embodiments of the disclosure.

Before undertaking the DETAILED DESCRIPTION below, it may beadvantageous to set forth definitions of certain words and phrases usedthroughout this patent document: the terms “include” and “comprise,” aswell as derivatives thereof, mean inclusion without limitation; the term“or,” is inclusive, meaning and/or; the phrases “associated with” and“associated therewith,” as well as derivatives thereof, may mean toinclude, be included within, interconnect with, contain, be containedwithin, connect to or with, couple to or with, be communicable with,cooperate with, interleave, juxtapose, be proximate to, be bound to orwith, have, have a property of, or the like; and the term “controller”means any device, system or part thereof that controls at least oneoperation, such a device may be implemented in hardware, firmware orsoftware, or some combination of at least two of the same. It should benoted that the functionality associated with any particular controllermay be centralized or distributed, whether locally or remotely.Definitions for certain words and phrases are provided throughout thispatent document, those of ordinary skill in the art should understandthat in many, if not most instances, such definitions apply to prior, aswell as future uses of such defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an example embodiment of a configuration of a LongTerm Evolution (LTE) system;

FIG. 2 illustrates an example embodiment of a radio protocolarchitecture in an LTE system;

FIG. 3 illustrates an example embodiment of Carrier Aggregation (CA) fora User Equipment (UE);

FIG. 4 illustrates an example embodiment of Time Division Duplexing(TDD) frame structure;

FIG. 5 illustrates an example embodiment of cells having different TDDUpLink/DownLink (UL/DL) configurations;

FIG. 6 illustrates an example embodiment of an operation of a UEaccording to this disclosure;

FIG. 7 is a flowchart illustrating an example embodiment of an operationof a UE in a subframe of a secondary serving cell according to thisdisclosure;

FIG. 8 is a flowchart illustrating an example embodiment of an operationof a UE for determining a feedback reception time and a dataretransmission time in a secondary serving cell according to thisdisclosure;

FIG. 9 is a flowchart illustrating an example embodiment of an operationof a UE for determining a feedback transmission time in a secondary cellaccording to this disclosure;

FIGS. 10A and 10B are flowcharts illustrating an example embodiment ofan operation of a UE for performing a UL transmission in a secondaryserving cell according to this disclosure;

FIG. 11 is a block diagram of an example embodiment of a UE according tothis disclosure;

FIG. 12 is a block diagram of an example embodiment of an evolved Node B(eNB) according to this disclosure;

FIGS. 13A and 13B illustrate example embodiments of situations in whichsubframes of serving cells are overlapped with each other;

FIG. 14 is a flowchart illustrating an example embodiment of anoperation of a UE in a subframe of a secondary serving cell according tothis disclosure;

FIG. 15 illustrates an example embodiment of a measurement gap accordingto this disclosure;

FIG. 16 illustrates an example embodiment of a problem involved inconfiguring a measurement gap, when the subframe boundaries of servingcells are not aligned;

FIG. 17 illustrates an example embodiment of setting of a measurementgap, when a DL reception time of a secondary serving cell precedes a DLreception time of a primary serving cell;

FIG. 18 illustrates an example embodiment of setting of a measurementgap, when a DL reception time of a primary serving cell precedes a DLreception time of a secondary serving cell;

FIG. 19 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 20 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 21 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 22 illustrates an example embodiment of a relationship between ameasurement gap and a UL subframe in a Frequency Division Duplexing(FDD) system;

FIG. 23 illustrates an example embodiment of timing adjustment of a ULsubframe in a TDD system;

FIG. 24 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to setting of a measurement gap accordingto this disclosure;

FIG. 25 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 26 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 27 illustrates an example embodiment of serving cells havingdifferent UL/DL configurations;

FIG. 28 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to setting of a measurement gap accordingto this disclosure;

FIG. 29 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 30 illustrates an example embodiment of subframe sets according tothis disclosure;

FIG. 31 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 32 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure;

FIG. 33 illustrates an example embodiment of setting of a measurementgap according this disclosure;

FIG. 34 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according thisdisclosure;

FIG. 35 is a diagram illustrating an example embodiment of a signal flowfor an overall operation for determining the number of bits in a RankIndicator (RI) according to this disclosure; and

FIG. 36 is a flowchart illustrating an example embodiment of anoperation of a UE for determining the number of bits in an RI accordingto this disclosure.

Throughout the drawings, like reference numerals will be understood torefer to like parts, components, and structures.

DETAILED DESCRIPTION

FIGS. 1 through 36, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless communication system. Thefollowing description with reference to the accompanying drawings isprovided to assist in a comprehensive understanding of exemplaryembodiments of the disclosure as defined by the claims and theirequivalents. It includes various specific details to assist in thatunderstanding but these are to be regarded as merely exemplary.Accordingly, those of ordinary skilled in the art will recognize thatvarious changes and modifications of the embodiments described hereincan be made without departing from the scope and spirit of thedisclosure. In addition, descriptions of well-known functions andconstructions may be omitted for clarity and conciseness.

The terms and words used in the following description and claims are notlimited to the bibliographical meanings, but, are merely used by theinventor to enable a clear and consistent understanding of thedisclosure. Accordingly, it should be apparent to those skilled in theart that the following description of exemplary embodiments of thepresent disclosure is provided for illustration purpose only and not forthe purpose of limiting the disclosure as defined by the appended claimsand their equivalents.

It is to be understood that the singular forms “a,” “an,” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, reference to “a component surface” includes referenceto one or more of such surfaces.

By the term “substantially” it is meant that the recited characteristic,parameter, or value need not be achieved exactly, but that deviations orvariations, including for example, tolerances, measurement error,measurement accuracy limitations and other factors known to those ofskill in the art, may occur in amounts that do not preclude the effectthe characteristic was intended to provide.

FIG. 1 illustrates an example embodiment of a configuration of a mobilecommunication system according to this disclosure. While the followingdescription is given in the context of a Long Term Evolution (LTE)system as an example of the mobile communication system to which thepresent disclosure is applied, it is clearly to be understood that thepresent disclosure is not limited to the specific system.

Referring to FIG. 1, a Radio Access Network (RAN) of the mobilecommunication system includes evolved Node Bs (eNBs, eNode Bs, Node Bs,or Base Stations (BSs)) 105, 110, 115, and 120, a Mobility ManagementEntity (MME) 125, and a Serving GateWay (serving-GW) 130. User Equipment(UE or Mobile Station (MS)) 135 is connected to an external network (notshown) through the eNBs 105, 110, 115, and 120 and the S-GW 130.

The eNBs 105, 110, 115, and 120 correspond to Node Bs of a UniversalMobile Telecommunication System (UMTS). An eNB is connected to the UE135 and plays a more complex role than a legacy Node B. Since all usertraffic including real-time service (such as Internet Protocol(IP)-based voice service or Voice over IP (VoIP)) is serviced through ashared channel in the LTE system, an entity that schedules UEs accordingto state information about the UEs is needed. State information aboutthe UEs can include buffer states, available transmission power states,and channel states of the UEs. Such entities are the eNBs 105, 110, 115,and 120. One eNB generally controls a plurality of cells.

To achieve a high data rate (such as 100 Mbps), the LTE system adoptsOrthogonal Frequency Division Multiplexing (OFDM) as a radio accesstechnology in a bandwidth of 20 MHz. In addition, the LTE system uses anAdaptive Modulation and Coding (AMS) scheme in which a modulation schemeand a channel coding rate are determined for a UE adaptively accordingto the channel state of the UE.

The S-GW 130 is configured to provide a data bearer and is configured togenerate or remove the data bearer under the control of the MME 125. TheMME 125, which is an entity that performs a mobility management functionand other control functions for the UE 135, is connected to theplurality of eNBs 105, 110, 115, and 120.

FIG. 2 illustrates an example embodiment of a radio protocolarchitecture in an LTE system according to this disclosure.

Referring to FIG. 2, the radio protocol architecture of the LTE systemincludes Packet Data Convergence Protocol (PDCP) layers 205 and 240,Radio Link Control (RLC) layers 210 and 235, Medium Access Control (MAC)layers 215 and 230, and Physical (PHY) layers 220 and 225 in a UE and aneNB. The PDCP layers 205 and 240 compress or decompress an IP header.The RLC layers 210 and 235 perform an Automatic Repeat reQuest (ARQ)operation by reconfiguring a PDCP Packet Data Unit (PDU) to anappropriate size. Each of the MAC layers 215 and 230 are connected to aplurality of RLC entities configured in the UE or the eNB. The MAClayers 215 and 230 multiplex RLC PDUs into a MAC PDU and demultiplex aMAC PDU into RLC PDUs.

The PHY layers 220 and 225 channel-encode or modulate higher layer dataof OFDM symbols and transmit the OFDM symbols on a radio channel. ThePHY layers 220 and 225 also demodulate and channel-decode OFDM symbolsreceived on a radio channel or provide the channel-decoded OFDM symbolsto a higher layer or perform a Hybrid ARQ (HARQ) operation for datatransmission or reception. To support UpLink (UL) data transmission, thePHY layers 220 and 225 use a Physical Uplink Shared Channel (PUSCH), aPhysical HARQ Indicator Channel (PHICH) carrying an ACKnowledgement orNegative ACKnowledgement (ACK/NACK) as an HARQ feedback for a PUSCHtransmission, a Physical Downlink Control Channel (PDCCH) carrying aDownLink (DL) control signal (e.g. scheduling information), or aPhysical Uplink Control Channel (PUCCH) carrying a UL control signal.Further, the PHY layers 220 and 225 can use a Physical Downlink SharedChannel (PDSCH) to support DL data transmission.

FIG. 3 illustrates an example embodiment of Carrier Aggregation (CA) fora UE according to this disclosure.

Referring to FIG. 3, an eNB 305 generally transmits and receivesmultiple carriers across a plurality of frequency bands. If the eNB 305transmits a carrier 315 having a center frequency (f1) and a carrier 310having a center frequency (f3), a UE that is not CA-enabled can transmitor receive data in one of the two carriers 310 and 315. On the otherhand, a CA-enabled UE 330 can transmit or receive data simultaneously inthe plurality of carriers 310 and 315. When needed, the eNB 305 canallocate more carriers to the CA-enabled 330, thereby increasing thetransmission rate of the UE 330.

If one DL carrier and one UL carrier transmitted from or received at aneNB form one cell, CA can be regarded as a UE's simultaneoustransmission or reception of data in a plurality of cells. A maximumtransmission rate of data is increased in proportion to the number ofaggregated carriers.

In the following description of embodiments of the present disclosure,data reception in a DL carrier at a UE or data transmission in a ULcarrier from the UE means data transmission or reception on a controlchannel and a data channel provided by a cell corresponding to thecenter frequency and frequency band of the DL and UL carriers.Particularly, CA is regarded as configuration of a plurality of servingcells for a UE in the present disclosure. Each serving cell can be aPrimary serving Cell (PCell) or a Secondary serving Cell (SCell). In CA,one PCell and one or more SCells can be configured for a UE. These termsare used as defined by the LTE standard. For details, refer to 3GPP TS36.331 and 3GPP TS 36.321 (December 2011).

While the embodiments of the present disclosure will be described in thecontext of an LTE system for the convenience of description, theembodiments of the present disclosure are applicable to any wirelesscommunication system supporting CA.

In Time Division Duplexing (TDD) mode, one frequency band is used for aUL during the duration of a specific subframe and for a DL during theduration of another subframe. A UE should have accurate knowledge of thepositions of UL subframes and DL subframes and an eNB preliminarilytransmits information about the positions of these UL and DL subframesto the UE.

Information about UL subframes and DL subframes is referred to as a TDDUL/DL configuration. TABLE 1 lists TDD UL/DL configurations that an eNBcan provide according to this disclosure. Each subframe is configured asa UL subframe, a DL subframe, or a special subframe according to a TDDUL/DL configuration.

TABLE 1 Downlink- UL/DL to-Uplink Config- Switch-point Subframe numberuration periodicity 0 1 2 3 4 5 6 7 8 9 0 5 ms D S U U U D S U U U 1 5ms D S U U D D S U U D 2 5 ms D S U D D D S U D D 3 10 ms  D S U U U D DD D D 4 10 ms  D S U U D D D D D D 5 10 ms  D S U D D D D D D D 6 5 ms DS U U U D S U U D

In TABLE 1, ‘D’ represents a DL subframe used to transmit DL data, ‘U’represents a UL subframe used to transmit UL data, and ‘S’ represents aspecial subframe interposed between a DL subframe and a UL subframe.

The reason for configuring a special subframe is that UEs differ in thetiming of completely receiving a DL subframe and the timing oftransmitting a UL subframe depending on the positions of the UEs. Forexample, a UE remote from an eNB receives data from the eNB later than aUE near to the eNB. Therefore, for the eNB to receive data from theremote UE within a specific time, the remote UE should start datatransmission earlier than the nearby UE. That is, a certain Guard Period(GP) is needed between DL data reception and UL data transmission anddefined in the special subframe. On the contrary, a special subframe isnot needed between a UL subframe and a DL subframe.

FIG. 4 illustrates an example embodiment of a TDD frame structureaccording to this disclosure.

Referring to FIG. 4, a 10-ms radio frame 400 is divided into 10subframes. Each radio frame 400 is identified by a System Frame Number(SFN) such as an integer ranging from 0 to 4095. Each time one radioframe elapses, the SFN can be incremented by 1. Each subframe 405 can be1 ms long, including two slots. A special subframe can be divided intothree parts, a Downlink Pilot Time Slot (DwPTS) 410, a GP 425, and anUplink Pilot Time Slot (UpPTS) 420. The DwPTS 410 is a time period forDL reception and the UpPTS 420 is a time period for UL transmission. Notransmission or reception can take place in the GP 425.

Optimum lengths of the DwPTS 410 and the UpPTS 420 vary with propagationenvironments. An eNB can indicate appropriate lengths of the DwPTS 410and the UpPTS 420 in advance to a UE. The eNB broadcasts, to UEs, theTDD UL/DL configurations listed in (Table 1) and the lengths of theDwPTS and the UpPTS listed in TABLE 2 in a TDD-configured InformationElement (IF) of System Information Block Type 1 (SIB1).

TABLE 2 Normal cyclic prefix in downlink Extended cyclic prefix indownlink UpPTS UpPTS Normal Extended Normal Extended Special subframecyclic prefix cyclic prefix cyclic prefix cyclic prefix in configurationDwPTS in uplink in uplink DwPTS in uplink uplink 0  6592Ts 2192Ts 2560Ts 7680Ts 2192Ts 2560Ts 1 19760Ts 20480Ts 2 21952Ts 23040Ts 3 24144Ts25600Ts 4 26336Ts  7680Ts 4384Ts 5120Ts 5  6592Ts 4384Ts 5120Ts 20480Ts6 19760Ts 23040Ts 7 21952Ts — — — 8 24144Ts — — —

As noted in TABLE 2, the lengths of the DwPTS and the UpPTS can be setto multiples of a symbol duration, Ts, according to special subframeconfigurations indicated by the eNB and the types of Cyclic Prefixes(CPs) used for the DL and the UL.

If one eNB provides a plurality of TDD serving cells, compatibility witha legacy system can be ensured and better load balancing can be enabledby applying different TDD UL/DL configurations to the plurality of TDDserving cells. For example, in the case where a TDD cell of a 3G mobilecommunication system and a TDD cell of an LTE mobile communicationsystem are provided over the same area and the operation frequencies ofthe two cells are adjacent, interference between the 3G TDD cell and theLTE TDD cell can be minimized by applying a specific UL/DL configurationto the LTE TDD cell. At the same time, a different UL/DL configurationfrom the LTE TDD cell can be applied to an LTE TDD cell having anoperation frequency very remote from the operation frequency of the 3GTDD cell, thereby increasing efficiency.

When different UL/DL configurations (such as a plurality of UL/DLconfigurations) are applied to a plurality of serving cells of the sameeNB as described above, UEs connected to the serving cells canexperience different types of subframes between the serving cells duringa predetermined time period.

FIG. 5 illustrates an example embodiment of cells having different TDDUL/DL configurations according to this disclosure. Referring to FIG. 5,if a first serving cell (serving cell 1) and a second serving cell(serving cell 2) are configured for a UE and UL/DL configuration 1 andUL/DL configuration 2 are applied to serving cell 1 and serving cell 2,respectively, a UL subframe (hereinafter, referred to as a “U subframe”)of serving cell 1 and a DL subframe (hereinafter, referred to as a “Dsubframe”) of serving cell 2 coincide during a time period 505. Althoughit is preferred that the UE performs UL transmission in serving cell 1and DL reception in serving cell 2 during the time period 505, the ULtransmission and the DL reception can be impossible under circumstances.

Basically, a UE does not need to perform UL transmission and DLreception simultaneously in a TDD system. Accordingly, a Radio Frequency(RF) circuit of the TDD UE is designed to perform only one of a DLoperation and a UL operation during one time period, that is, to operatein Half Duplex (HF). Thus, a general TDD UE that does not have a FullDuplex (FD) function of simultaneous UL and DL operations is allowed toperform only one of a DL reception and a UL transmission during the timeperiod 505.

In this context, embodiments of the present disclosure provide methodsand apparatuses for performing reliable communication by enabling an eNBand a UE to operate in the same link direction in the above situation.

According to the embodiments of the present disclosure, the UE and theeNB can operate as follows.

-   -   If different UL/DL configurations are assigned to a PCell and an        SCell, an HD UE does not perform an operation related to an        subframe of the SCell (referred to as an “SCell subframe”)        directed to a different direction from a subframe of the PCell        (referred to as a “PCell subframe”).    -   If different UL/DL configurations are assigned to a PCell and an        SCell, an HD UE determines an SCell subframe in which the HD UE        will receive a PHICH for a PUSCH transmission, based on the        UL/DL configuration of the PCell.    -   If different UL/DL configurations are assigned to a PCell and an        SCell, an HD UE determines an SCell subframe in which the HD UE        will retransmit a PUSCH, based on the UL/DL configuration of the        PCell.

FIG. 6 illustrates an example embodiment of an operation of a UEaccording to this disclosure.

Referring to FIG. 6, serving cells having different TDD UL/DLconfigurations, such as PCell 603 and SCell 605 can be configured for aUE 601 by CA. The same eNB manages the PCell 603 and the SCell 605 whilethe UE 601 is initially connected only to the PCell 603.

The PCell 603 carries a UECapabilityEnquiry message to the UE 601 tocheck functions supported by the UE 601 in operation 611. That is, theeNB transmits the UECapabilityEnquiry message to the UE 601 in the PCell603. The UE 601 indicates its supported functions to the eNB bytransmitting a UECapabilityInformation message in the PCell 603 inoperation 613. The UE 601 reports its CA capability by setting bandcombinations supported by the UE 601 in a ‘supported band combination’Information Element (IE) of the UECapabilityInformation message.

The ‘supported band combination’ IE, includes one or more bandparameters. One of the band parameters includes a band indicator, thenumber and bandwidths of serving cells configurable in the bands of theband combination, configurable Multiple Input Multiple Output (MIMO)information, and the like. In addition, 1-bit information indicatingavailability of simultaneous transmission and reception is reported fora band combination satisfying a predetermined condition from among thesupported band combinations. The predetermined condition can be, forexample, that the band combination includes only TDD bands and the TDDbands are different.

For example, in the case where the UE reports its supported bandcombinations as combination 1 to band combination 10 listed in TABLE 3below, the UE sets 1-bit information indicating availability ofsimultaneous transmission and reception only for band combination 9 andband combination 10. In TABLE 3, ‘band FDD’ represents an LTE frequencyband in FDD and ‘band TDD’ represents an LTE frequency band in TDD. Forexample, band 1 to band 32 are FDD frequency bands and band 33 to band64 are TDD frequency bands among LTE frequency bands.

TABLE 3 Index of Report availability supported band Composition of bandof simultaneous combination combination transmission and receptionsupported band band FDD1 No combination 1 supported band band FDD2 Nocombination 2 supported band band TDD1 No combination 3 supported bandband TDD2 No combination 4 supported band band TDD3 No combination 5supported band band FDD1, band FDD1 No combination 6 supported band bandFDD1, band FDD2 No combination 7 supported band band TDD1, band TDD1 Nocombination 8 supported band band TDD1, band TDD2 Yes combination 9supported band band TDD1, band TDD3 Yes combination 10

In regards to each ‘supported band combination’ satisfying thepredetermined condition, the UE reports whether simultaneoustransmission and reception is available in the band combination by 1-bitinformation. The 1-bit information can be configured into a bitmap. Forexample, in TABLE 3 the UE reports a bitmap including 2-bit meaningfulinformation for band combination 9 and band combination 10. Each bit ofthe bitmap represents a band combination satisfying the predeterminedcondition in the order of inclusion in the UECapabilityInformationmessage. In the example of TABLE 3, the first bit and the second bit ofthe bitmap indicate availability of simultaneous transmission andreception in band combination 9 and band combination 10, respectively.

If the UE reports that simultaneous transmission and reception isavailable in a specific TDD band combination, the UE can perform DLreception and UL transmission simultaneously even though different UL/DLconfigurations are assigned to the band combination and the directionsof subframes are different during a time period according to the UL/DLconfigurations.

In contrast, if the UE reports that simultaneous transmission andreception is not available in a specific TDD band combination, the UEshould perform only one of DL reception and UL transmission, ifdifferent UL/DL configurations are assigned to the band combination andthe directions of subframes during a time period are different accordingto the UL/DL configurations.

The eNB applies a necessary setting to the UE based on theUECapabilityInformation message. For example, if the UE supports CA in aspecific band combination and there are serving cells operating in thebands of the band combination in the eNB, the eNB can configure CA forthe band combination for the UE. That is, the eNB can configure aplurality of serving cells corresponding to the band combination for theUE.

The eNB transmits a control message that configures a plurality of TDDserving cells to the UE 601 in operation 615. The control message can bean RRCConnectionReconfiguration message. The SCell 605 is added to theUE 601 by the control message. The UE 601 is aware of the UL/DLconfiguration of the PCell 603 by system information (e.g. SIB 1)broadcast in the PCell 603 and the UL/DL configuration of the SCell 605by the control message that configures the SCell 605.

The UE 601 performs an establishment procedure requested by the eNB(such as RRC connection establishment for the SCell 605) based on theRRCConnectionReconfiguration message and transmits anRRCConnectionReconfigurationComplete message indicating completion ofthe RRC connection establishment procedure to the eNB in operation 617.Then the eNB transmits an activation command for the configured SCell605 to the UE 601 at a specific time in operation 619. The activationcommand can be an Activation/deactivation MAC Control Element (CE)message.

The UE 601 refers to the UL/DL configuration of the PCell 603 todetermine an operation in the SCell 605 during a subsequent time periodin operation 625. Specifically, the UE 601 operates in every subframe asillustrated in TABLE 4. That is, if a PCell subframe direction definedby the UL/DL configuration of the PCell 603 is different from an SCellsubframe direction defined by the UL/DL configuration of the SCell 605,the UE 601 determines an operation to perform in the SCell 605 based onthe UL/DL configuration of the PCell 603.

TABLE 4 PCell subframe SCell subframe UE operation in PCell UE operationin SCell D D PDCCH/PHICH/PDSCH reception PDCCH/PHICH/PDSCH reception D SPDCCH/PHICH/PDSCH reception PDCCH/PHICH reception (DwPTS reception) D UPDCCH/PHICH/PDSCH reception No uplink transmission S D PDCCH/PHICHreception, UpPTS PDCCH/PHICH reception transmission S S PDCCH/PHICHreception, UpPTS PDCCH/PHICH reception, transmission conditional UpPTStransmission S U PDCCH/PHICH reception, UpPTS conditional SRStransmission transmission U D PUSCH/PUCCH transmission PDCCH/PHICH/PDSCHnon-reception U S PUSCH/PUCCH transmission UpPTS transmission U UPUSCH/PUCCH transmission PUSCH transmission

If a PCell subframe and an SCell subframe corresponding to a currenttime period are D subframes, the UE performs DL reception in the twoserving cells during the current time period (such as in the samesubframes of the PCell and the SCell). That is, the UE receives a PDCCHand a PHICH in the control regions of the current subframes of the twoserving cells and a PDSCH in the data regions of the current subframesof the two serving cells. Likewise, the eNB transmits, to the UE, thePDCCH and the PHICH in the control regions of the current subframes ofthe two serving cells and the PDSCH in the data regions of the currentsubframes of the two serving cells.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are a D subframe and an S subframe, respectively, the UEperforms a D-subframe operation which includes receiving aPDCCH/PHICH/PDSCH in the PCell during the current time period andperforms a DL operation which includes a DwPTS reception in the SCellduring the current time period. The DwPTS reception means that a DLsignal is received only in a predetermined starting time period of acurrent subframe on a time axis. Since the PDSCH can occupy up to thelast of the subframe, the UE can receive only the PDCCH and the PHICHexcept for the PDSCH. In the above situation, that is, when the PCellsubframe is a D subframe and the SCell subframe is an S subframe duringthe same time period, the eNB does not schedule a PDSCH for the UE inthe SCell subframe and transmits only the PDCCH and the PHICH in theDwPTS of the SCell subframe.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are a D subframe and a U subframe, respectively, the UEperforms a D-subframe operation, which includes receiving aPDCCH/PHICH/PDSCH in the PCell during the current time period, whereasthe UE performs no UL transmission in the SCell during the current timeperiod even though a UL transmission is scheduled in the SCell.Therefore, the eNB does not perform a UL reception in this situation.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are an S subframe and a D subframe, respectively, the UEperforms an S-subframe operation in the PCell during the current timeperiod. That is, the UE receives a PDCCH/PHICH in the DwPTS of the PCellsubframe, discontinues transmission and reception in the GPS of thePCell subframe, and when needed, performs a UL transmission (such as aSounding Reference Signal (SRS)) in the UpPTS of the PCell subframe. Inaddition, the UE receives a DL signal in the SCell subframe only duringa time period overlapped between the DwPTS of the PCell subframe and theSCell subframe. That is, the UE receives a PDCCH and a PHICH in theSCell, except for a PDSCH. In this situation, when the PCell subframe isan S subframe and the SCell subframe is a D subframe, the eNB does notschedule a PDSCH in the SCell subframe and performs only a DLtransmission in the DwPTS of the PCell subframe.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are S subframes, the UE performs an S-subframe operation inthe PCell during the current time period. That is, the UE receives aPDCCH/PHICH in the DwPTS of the PCell subframe, discontinuestransmission and reception in the GP of the PCell subframe, and whenneeded, performs a UL transmission in the UpPTS of the PCell subframe,for example, transmits an SRS in the UpPTS. In addition, the UE receivesa DL signal in the SCell subframe only during a time period overlappedbetween the DwPTS of the PCell subframe and the SCell subframe. That is,the UE receives a PDCCH and a PHICH in the SCell subframe, except for aPDSCH. The UE discontinues transmission and reception in the SCellsubframe during a time period overlapped between the GP of the PCellsubframe and the SCell subframe. When needed, the UE can perform a ULtransmission in the UpPTS of the SCell subframe overlapped with theUpPTS of the PCell subframe. The UL transmission can be performeddepending on how much of the UpPTS of the SCell subframe is overlappedwith the UpPTS of the PCell subframe. Accordingly, the UL transmissionis optional. In this situation, the eNB can attempt to receive an SRS inthe UpPTS of the SCell subframe, transmit a PHICH in the DwPTS of theSCell subframe, and recognize that no transmission and reception of theUE occurs in the GP of the SCell subframe.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are an S subframe and a U subframe, respectively, the UEperforms an S-subframe operation in the PCell during the current timeperiod, while transmitting an SRS conditionally without transmitting aPUCCH and a PUSCH in the SCell during the current time period. That is,if SRS transmission is scheduled in the SCell U subframe and a part ofthe SCell U subframe overlapped with the UpPTS of the PCell S subframeis equal to or larger than a predetermined value (such as one OFDMsymbol period), the UE transmits an SRS in the SCell U subframe andotherwise, the UE does not transmit the SRS. In this situation, the eNBattempts to receive the SRS from the UE during the time periodoverlapped between the SCell U subframe and the UpPTS of the PCell Ssubframe.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are a U subframe and a D subframe, respectively, the UEperforms a U-subframe operation in the PCell subframe. That is, if aPUCCH or PUSCH transmission is scheduled in the PCell, the UE transmitsa PUCCH or a PUSCH in the PCell subframe. However, in this case, the UEdoes not receive a DL signal in the SCell during the current timeperiod. That is, the UE does not receive a PDCCH/PHICH/PDSCH in theSCell subframe. In this situation, when the PCell subframe is a Usubframe and the SCell subframe is a D subframe, the eNB does notschedule a PDSCH in the SCell.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are a U subframe and an S subframe, respectively, the UEperforms a U-subframe operation in the PCell during the current timeperiod, while when needed, the UE transmits a UL signal in the UpPTS ofthe SCell subframe. That is, if an SRS transmission is scheduled in theSCell S subframe, the UE transmits an SRS and does not receive a DLsignal in the DwPTS of the SCell S subframe. In this situation, the eNBattempts to receive the SRS from the UE during a time period overlappedbetween the UpPTS of the SCell S subframe and the PCell U subframe anddoes not transmit a DL signal to the UE in the DwPTS of the SCell Ssubframe.

If a PCell subframe and an SCell subframe corresponding to a currenttime period are U subframes, the UE performs a U-subframe operation inboth the PCell and the SCell during the current time period. That is,the UE transmits a PUSCH and a PUCCH, when needed. In this situation,the eNB attempts to receive the PUSCH from the UE in the SCell Usubframe.

In operation 630, the UE 601 determines a PUSCH transmission time basedon the UL/DL configuration of the PCell 603. The PUSCH transmission timerefers to the position of a subframe to carry a PUSCH. The UE 601generally refers to the UL/DL configuration of a serving cell intransmitting a PUSCH in the serving cell. For example, upon receipt of aPDCCH indicating a PUSCH transmission or an HARQ NACK signal in subframen, the UE transmits a PUSCH in subframe n+m where n and m are definedfor each UL/DL configuration.

An FD UE or a UE operating in CA under the same UL/DL configurationdetermines a PUSCH transmission time, an HARQ feedback reception time,and a PUSCH retransmission time, based on the UL/DL configuration of aserving cell. An HD UE operating under a plurality of UL/DLconfigurations determines a timing relationship between serving cellsbased on the UL/DL configuration of a PCell, instead of the UL/DLconfiguration of a serving cell in which a PUSCH will be transmitted.

The UE 601 refers to the UL/DL configuration of the PCell 603 indetermining a subframe in which an HARQ feedback will be received for aPUSCH transmitted in the SCell in operation 635. This is because if asubframe to carry a PHICH, determined based on the UL/DL configurationof the SCell, is overlapped with a U subframe of the PCell, the PHICHcan not be received.

FIG. 7 is a flowchart illustrating an example embodiment of an operationof a UE in an SCell subframe according to this disclosure.

Referring to FIG. 7, the UE receives system information about a servingcell and acquires the UL/DL configuration of the serving cell from thesystem information in operation 700. The UE then starts an RRCconnection establishment procedure for the serving cell. Once the RRCconnection establishment procedure is completed, the serving cellbecomes a PCell for the UE.

In operation 705, the UE reports its capabilities to an eNB by a UEcapability information message. The UE capability information messageincludes information about UE-supported band combinations and the UEadditionally reports information indicating whether simultaneoustransmission and reception is available in a band combination composedof different TDD bands (referred to as an “inter-TDD band combination”).

In operation 710, an SCell is configured for the UE. The SCell isconfigured by transmitting an RRC control message including SCellconfiguration information from the eNB to the UE and establishing asignal path for the SCell based on the SCell configuration informationby the UE. The SCell configuration information includes, for example,information about a center frequency of the SCell, radio resourcesallocated to the UE in the SCell, and the UL/DL configuration of theSCell. Each SCell can be placed in an active or inactive state. Datatransmission and reception is possible only in an active SCell. When anSCell is initially configured, the SCell is in an inactive state. Thenthe SCell is switched to an active state by a command from the eNB.

In operation 715, the UE waits until subframe n of the SCell starts inorder to determine an operation to perform in subframe n of the SCell.

The UE checks whether the following conditions are satisfied inoperation 720.

-   -   The PCell and the SCell have different frequency bands        (inter-band TDD carrier aggregation).    -   The PCell and the SCell have different UL/DL configurations.

If the following conditions are satisfied, the UE goes to operation 730and otherwise, the UE goes to operation 725. If the above conditions arenot satisfied, this implies that the PCell and the SCell have the samefrequency band or that even though PCell and the SCell have differentfrequency bands, they have the same UL/DL configuration. If the PCelland the SCell have the same frequency band, the same UL/DL configurationshould be assigned to the PCell and the SCell.

In operation 725, the UE determines the type of SCell subframe n basedon the UL/DL configuration of the SCell and determines an operation toperform according to the type of SCell subframe n. The UE operates asfollows according to the types of subframes.

-   -   If subframe n is a D subframe, the UE determines to receive a        PDCCH/PHICH/PDSCH in subframe n.    -   If subframe n is an S subframe, the UE determines to receive a        PDCCH/PHICH and when needed, to perform a UpPTS transmission in        subframe n.    -   If subframe n is a U subframe, the UE determines to transmit a        PUSCH and an SRS in subframe n, when needed.

The above operations may be limited by another solution or a requirementimposed on the UE. For example, if subframe n does not correspond to anactive time in a Discontinuous Reception (DRX) operation or overlapswith a Measurement Gap (MG) which is a time period for measuringnon-serving frequencies, the above operations may not be performed.During the measurement gap, the UE suspends an uplink transmission and adownlink reception, and then can perform a measurement of non-servingfrequencies.

On the other hand, if the above conditions are satisfied, this impliesthat the PCell and the SCell have different frequency bands anddifferent UL/DL configurations. In operation 730, the UE determineswhether simultaneous transmission and reception is available in acurrent band combination (such as the combination of the frequency bandsof the PCell and the SCell). Or the UE determines whether informationindicating that simultaneous transmission and reception is available inthe current band combination has been reported to the eNB. If thedetermination result is affirmative, the UE goes to operation 725 andotherwise, the UE goes to operation 735.

In operation 735, the UE determines an operation to perform in SCellsubframe n based on the type of a PCell subframe overlapped with SCellsubframe n. Since the frame boundaries of the PCell and the SCell aregenerally aligned, PCell subframe n is overlapped with SCell subframe n.

FIG. 8 illustrates a flowchart illustrating an example embodiment of anoperation of a UE for determining a feedback reception time and a PUSCHretransmission time in an SCell according to this disclosure.

Referring to FIG. 8, the UE receives system information about a servingcell and thus acquires the UL/DL configuration of the serving cell fromthe system information in operation 800. The UE then starts an RRCconnection establishment procedure for the serving cell. Once the RRCconnection establishment procedure is completed, the serving cellbecomes a PCell for the UE.

In operation 805, the UE reports its capabilities to an eNB by a UEcapability information message. The UE capability information messageincludes information about UE-supported band combinations and the UEadditionally reports information indicating availability of simultaneoustransmission and reception in an inter-TDD band combination.

In operation 810, an SCell is configured for the UE. The SCell isconfigured by transmitting an RRC control message including SCellconfiguration information from the eNB to the UE and establishing asignal path for the SCell based on the SCell configuration informationby the UE. The SCell configuration information includes, for example,information about a center frequency of the SCell, radio resourcesallocated to the UE in the SCell, and the UL/DL configuration of theSCell.

In operation 815, the UE transmits a PUSCH in SCell subframe n. Then theUE proceeds to operation 820 to determine a subframe in which an HARQfeedback for the PUSCH transmission will be received and a subframe inwhich the PUSCH will be retransmitted in case the HARQ feedback is aNACK.

The UE checks whether the following conditions are satisfied inoperation 820.

-   -   The PCell and the SCell have different frequency bands        (inter-band TDD carrier aggregation).    -   The PCell and the SCell have different UL/DL configurations.

If the following conditions are satisfied, the UE goes to operation 830and otherwise, the UE goes to operation 825. If the above conditions arenot satisfied, this implies that the PCell and the SCell have the samefrequency band or that even though PCell and the SCell have differentfrequency bands, they have the same UL/DL configuration. If the PCelland the SCell have the same frequency band, the same UL/DL configurationshould be assigned to the PCell and the SCell. In operation 825, the UEdetermines the position of a subframe in which to receive a PHICH forthe PUSCH transmitted in SCell subframe n and the position of a subframefor PUSCH retransmission, based on the UL/DL configuration of the SCell.The PHICH reception time and the PUSCH retransmission time are definedaccording to a UL/DL configuration in the standard. For example, if theUE receives a NACK as an HARQ feedback in subframe n, the UE retransmitsa PUSCH in subframe n+k. For example, k is determined according to theUL/DL configuration and the index of the subframe in which the PHICH hasbeen received, as illustrated in TABLE 5.

TABLE 5 TDD UL/DL subframe number n Configuration 0 1 2 3 4 5 6 7 8 9 04 6 4 6 1 6 4 6 4 2 4 4 3 4 4 4 4 4 4 5 4 6 7 7 7 7 5

If the above conditions are satisfied, this implies that the PCell andthe SCell have different frequency bands and different UL/DLconfigurations. In operation 830, the UE determines whether simultaneoustransmission and reception is available in a current band combination.Or the UE determines whether information indicating that simultaneoustransmission and reception is available in the current band combinationhas been reported to the eNB. If the determination result isaffirmative, the UE goes to operation 825 and otherwise, the UE goes tooperation 835.

In operation 835, the UE determines the PHICH reception time and thePUSCH retransmission time based on the UL/DL configuration of the PCell,instead of the UL/DL configuration of the SCell. For example, if UL/DLconfiguration 4 is assigned to the SCell, UL/DL configuration 6 isassigned to the PCell, and the UE receives an HARQ feedback, NACK insubframe 9, the UE selects 5 as the value of k according to the UL/DLconfiguration of the PCell in TABLE 5. Then the UE retransmits the PUSCHin subframe (9+5), that is, subframe 4 of the next radio frame.

Likewise, the eNB transmits an HARQ feedback to the UE that has reportedthat simultaneous transmission and reception is not available in theSCell having a different UL/DL configuration from the PCell andconsiders the UE's decision on a PHICH reception time and a PUSCHretransmission time based on the UL/DL configuration of the PCell inmanaging PUSCH transmission resources. That is, the eNB determines totransmit the HARQ feedback, NACK in subframe 9 of the SCell based on theUL/DL configuration of the PCell configured for the UE and determines toreceive retransmission data on the PUSCH in subframe (9+5) of the SCell,that is, subframe 4 of the next radio frame in the SCell.

FIG. 9 is a flowchart illustrating an example embodiment of an operationof a UE for determining a feedback transmission time in an SCellaccording to this disclosure. Operations 900, 905, and 910 are performedin the same manner as operations 800, 805, and 810 illustrated in FIG. 8and thus will not be described herein in detail.

Referring to FIG. 9, a PDCCH indicating a PDSCH transmission (or a DLtransmission) is received or a subframe for which a DL assignment isconfigured comes in operation 915. The UE receives PDSCH data in thesubframe, decodes the PDSCH data, and proceeds to operation 920 totransmit a feedback for the PDSCH data.

In operation 920, the UE determines whether a serving cell carrying thePDSCH data is a PCell or an SCell. If the serving cell carrying thePDSCH data is the PCell, the UE goes to operation 925 and if the servingcell carrying the PDSCH data is the SCell, the UE goes to operation 930.

In operation 930, the UE determines whether the following conditions aresatisfied. If the conditions are satisfied, the UE goes to operation 940and otherwise, the UE goes to operation 935.

If the conditions are not satisfied, this implies that the PCell and theSCell have the same frequency band or despite different frequency bands,the PCell and the SCell have the same UL/DL configuration. If the PCelland the SCell have the same frequency band, the same UL/DL configurationshould be assigned to the two serving cells. In operation 935, the UEselects a subframe in which the HARQ feedback will be transmitted, basedon the UL/DL configuration of a current time period. If the UE receivesa PDSCH in subframe n−k, the UE transmits an HARQ feedback for the PDSCHin subframe n. For example, k is defined according to the number n of asubframe to carry an HARQ feedback according to a UL/DL configuration,as illustrated in TABLE 6 below.

TABLE 6 UL/DL Subframe n Configuration 0 1 2 3 4 5 6 7 8 9 0 — — 6 — 4 —— 6 — 4 1 — — 7, 6 4 — — — 7, 6 4 — 2 — — 8, 7, 4, 6 — — — — 8, 7, 4, 6— — 3 — — 7, 6, 11 6, 5 5, 4 — — — — — 4 — — 12, 8, 7, 11 6, 5, 4, 7 — —— — — — 5 — — 13, 12, 9, 8, 7, 5, 4, — — — — — — — 11, 6 6 — — 7 7 5 — —7 7 —

For example, if the UL/DL configuration is UL/DL configuration 5 and nis 2, k is one of 13, 12, 9, 8, 7, 5, 3, 11, and 6.

If the conditions are satisfied, this implies that the PCell and theSCell have different frequency bands and different UL/DL configurations.In operation 940, the UE determines whether simultaneous transmissionand reception is available in a current band combination. Or the UEdetermines whether information indicating that simultaneous transmissionand reception is available in the current band combination has beenreported to the eNB. If the determination result is affirmative, the UEproceeds to operation 945 and otherwise, the UE proceeds to operation925.

In operation 925, the UE selects k based on the UL/DL configuration ofthe PCell instead of the UL/DL configuration of a serving cell to carrythe HARQ feedback and selects a subframe to carry the HARQ feedbackaccording to the value of k.

If the UE proceeds from operation 940 to operation 945, this means thatthe UE may perform simultaneous transmission and reception. In operation945, the UE selects k, taking into account the UL/DL configurations ofboth the PCell and the SCell and selects a subframe to carry the HARQfeedback according to the selected k.

Considering the UL/DL configurations of both the PCell and the SCellamounts to identifying a predetermined reference UL/DL configurationaccording to the combination of the two UL/DL configurations andselecting a subframe to carry the HAR feedback based on the referenceUL/DL configuration. If a UE capable of simultaneous transmission andreception considers only the UL/DL configuration of one of a PCell andan SCell, a potential maximum performance that can be achieved by the UEis unnecessarily restricted. Therefore, it is preferred to use areference UL/DL configuration predefined in the standard. For eachcombination of UL/DL configurations of a PCell and an SCell, an optimumUL/DL configuration can be determined as a reference UL/DL configurationand clarified in the standard. For example, if UL/DL configuration 3 andUL/DL configuration 1 are assigned to the PCell and the SCell,respectively, their reference UL/DL configuration can be set to UL/DLconfiguration 4.

Once the value of k is determined, the UE activates an HARQ Round TripTime (RTT) timer to the sum of 4 and k determined in operation 935 oroperation 945 in operation 950 and ends the procedure. The UE waitsuntil receiving a PDCCH indicating a DL transmission according to theHARQ RTT timer.

The HARQ RTT timer is defined so that a UE operating in DRX mode candiscontinue PDCCH monitoring to save battery power until receiving anHARQ retransmission. Unless otherwise needed, the UE can discontinuePDCCH monitoring while the HARQ RTT timer is running. If the HARQ RTTtimer expires and the UE fails to decode data of an HARQ processassociated with the HARQ RTT timer, the UE activates adrx-Retransmission timer.

As described before, if serving cells having different TDD bands anddifferent UL/DL configurations are configured for a UE that does notsupport simultaneous transmission and reception in the band combinationof the TDD bands and the UE receives a PDSCH in an SCell, the UE selectsa subframe to carry an HARQ feedback based on the UL/DL configuration ofa PCell.

FIGS. 10A and 10B are flowcharts illustrating an example embodiment ofan operation of a UE for performing a UL transmission in an SCellaccording to this disclosure. In FIGS. 10A and 10B, a UE determines aPHICH reception time and a PUSCH transmission time based on the UL/DLconfiguration of an SCell instead of the UL/DL configuration of a PCell.Therefore, the UE may not receive a PHICH or may not transmit a PUSCH.Herein, the UE increases transmission efficiency by setting relatedvariables to optimum values. Operations 1005 and 1010 are performed inthe same manner as operations 905 and 910 illustrated in FIG. 9 and thuswill not be described herein in detail.

Referring to FIGS. 10A and 10B, the UE receives a control messageindicating configuration of an SCell and configures the SCell inoperation 1015. The frequency band and UL/DL configuration of the SCellare different from the frequency band and UL/DL configuration of thePCell.

In operation 1020, the UE determines that it is necessary to perform aUL transmission in subframe n of the SCell. For example, the UE canreceive a UL grant for subframe n or an SRS transmission can bescheduled in subframe n.

In operation 1025, the UE determines whether simultaneous transmissionand reception is available in a current band combination or informationindicating that simultaneous transmission and reception is available hasbeen reported to an eNB. If the determination result is affirmative, theUE proceeds to operation 1085 and otherwise, the UE goes to operation1030.

In operation 1030, the UE determines whether an SRS or a PUSCH is to betransmitted. In the case of an SRS, the UE goes to operation 1035 and inthe case of a PUSCH, the UE goes to operation 1045.

In operation 1035, the UE determines whether subframe n of the PCell isa D subframe, an S subframe, or a U subframe. If PCell subframe n is a Dsubframe, the UE does not transmit the SRS in the SCell and waits untila UL transmission is needed again in the SCell in operation 1040.

If PCell subframe n is an S subframe, the UE transmits the SRS in theSCell conditionally in operation 1042. Specifically, if the time periodof the last OFDM symbol of SCell subframe n is fully included in thetime period of the UpPTS of PCell subframe n, the UE transmits the SRSand otherwise, the UE does not transmit the SRS and waits until a ULtransmission is needed again in the SCell.

If PCell subframe n is a U subframe, the UE transmits the SRS in theSCell and waits until a UL transmission is needed again in the SCell inoperation 1043.

In operation 1045, the UE determines whether PCell subframe n a Dsubframe, an S subframe, or a U subframe. If PCell subframe n is a Dsubframe or an S subframe, the UE does not transmit the PUSCH inoperation 1050 and increases CURRENT_TX_NB by 1 and maintainsCURRENT_IRV in operation 1060. CURRENT_TX_NB and CURRENT_IRV arevariables related to PUSCH transmission and have the following meanings.

CURRENT_TX_NB: a variable indicating the number of transmissions of apacket on a PUSCH in a current HARQ operation. If CURRENT_TX_NB reachesa predetermined maximum value, the UE deletes the packet from a buffer.

CURRENT_IRV: a variable indicating the Redundancy Version (RV) of thepacket in the current HARQ operation. The UE applies an RV indicated byCURRENT_IRV to PUSCH transmission. RV indicates a configuration of codedbits to be included in a packet to be transmitted on the PUSCH.

Each time the UE receives an HARQ feedback or transmits a PUSCH, the UEupdates these variables. If the eNB determines that the UE will nottransmit a PUSCH for any reason, it is preferred that the UE and the eNBmaintain CURRENT_IRV. Each time a non-adaptive retransmission isperformed in a UL HARQ operation, a specific RV is automaticallyapplied. The non-adaptive retransmission refers to a retransmission ofthe UE in the same transmission resources as used for a previoustransmission. Upon receipt of a NACK signal as a feedback, the UEbasically performs a non-adaptive retransmission. For example, the UEapplies RV 0 to an initial transmission, RV 2 to a first non-adaptiveretransmission, RV 3 to a second non-adaptive retransmission, and RV 1to a third non-adaptive retransmission. The UE and the eNB determine anRV for a next retransmission based on CURRENT_IRV. If CURRENT_IRV isincreased in spite of non-transmission of the PUSCH, a part of the RVsis not used in the PUSCH transmission, thereby degrading performance.Thus, the UE maintains CURRENT_IRV in operation 1060.

CURRENT_TX_NB is used to prevent further retransmission in the casewhere PUSCH transmission is failed despite a predetermined number of ormore transmissions. If the current number of transmissions of a currentpacket reaches a predetermined maximum value, the UE deletes the packetfrom an HARQ buffer and does not retransmit the packet any longer. Ifthe current number of transmissions of the current packet reaches thepredetermined maximum value, the eNB can allocate time/frequencyresources allocated to the UE for packet transmission to another UE,considering that the packet will not be retransmitted non-adaptively anylonger. Accordingly, it is important for the UE and the eNB to determinethat the number of transmissions of a current packet has reached apredetermined maximum number at the same time. For this purpose, the UEand the eNB preferably manage CURRENT_TX_NB based on the number ofelapses of retransmission time points, not based on the number of actualPUSCH transmissions. That is, each time a transmission time of a packetelapses, the UE and the eNB preferably increase CURRENT_TX_NB by 1 eventhough the packet is not actually transmitted.

After setting CURRENT_TX_NB and CURRENT_IRV in operation 1060, the UEsets HARQ_FEEDBACK to NACK without receiving a PHICH in operation 1077.HARQ_FEEDBACK is a variable indicating an HARQ feedback for a PUSCHtransmission. If HARQ_FEEDBACK is NACK, the UE performs a retransmissionat a next retransmission time. If HARQ_FEEDBACK is ACK, the UE does notperform a retransmission at a next retransmission time.

This is because PHICH reception despite non-transmission of PUSCH datajust increases an HARQ feedback error probability. Because the UE hasnot transmitted PUSCH data, there is no possibility that the eNBreceives PUSCH data without an error. Accordingly, HARQ_FEEDBACK is setto NACK.

If PCell subframe n is a U subframe in operation 1045, the UE transmitsPUSCH data in SCell subframe n in operation 1065 and increases each ofCURRENT_TX_NB and CURRENT_IRV by 1 in operation 1067. In operation 1068,the UE determines a subframe in which to receive a PHICH based on theUL/DL configuration of the SCell. The UE determines whether a PCellsubframe corresponding to the PHICH reception subframe is a U subframein operation 1070. In the case of a U subframe, the UE proceeds tooperation 1075. In the case of a D subframe or an S subframe, the UEgoes to operation 1080.

In operation 1075, the UE does not receive a PHICH and setsHARQ_FEEDBACK to ACK because although the UE has transmitted the PUSCH,the UE has not received a feedback for the PUSCH. If the UE setsHARQ_FEEDBACK to NACK in this case, the UE performs an unnecessary PUSCHretransmission.

In operation 1080, the UE receives a PHICH and sets HARQ_FEEDBACK to ACKor NACK according to a feedback received on the PHICH.

If simultaneous transmission and reception is available in the currentband combination or information indicating support of simultaneoustransmission and reception in the current band combination has beenreported to the eNB in operation 1025, the UE transmits a PUSCH in SCellsubframe n in operation 1085 and increases each of CURRENT_TX_NB andCURRENT_IRV by 1 in operation 1090. In operation 1095, the UE determinesa subframe in which to receive a PHICH based on the UL/DL configurationof the SCell. Then the UE receives a feedback on the PHICH in thedetermined subframe and sets HARQ_FEEDBACK to ACK or NACK according tothe received feedback in operation 1080.

Only when a UL transmission is not restricted by another solution or arequirement, an SRS or a PUSCH is transmitted in operations 1042, 1043,1065, and 1085.

FIG. 11 is a block diagram of an example embodiment of a UE according tothis disclosure.

Referring to FIG. 11, the UE includes a transceiver 1105, a controller1110, a Multiplexer (MUX) and Demultiplexer (DEMUX) unit 1120, a controlmessage processor 1135, and one or more higher layer processors 1125 and1130.

The transceiver 1105 includes a receiver that receives data and acontrol signal on a DL channel of a serving cell and a transmitter thattransmits data and a control signal on a UL channel of the serving cell.If a plurality of serving cells are configured for the, the transceiver1105 can transmit and receive data and a control signal in the pluralityof serving cells.

The MUX and DEMUX unit 1120 multiplexes data generated from the higherlayer processors 1125 and 1130 or the control message processor 1135 ordemultiplexes data received from the transceiver 1105 and provides thedemultiplexed data to the higher layer processors 1125 and 1130 or thecontrol message processor 1135.

The control message processor 1135 processes a control message receivedfrom an eNB and performs a necessary operation according to theprocessed control message. For example, upon receipt of an SCell-relatedparameter, the control message processor 1135 provides the SCell-relatedparameter to the controller 1110.

The higher layer processors 1125 and 1130 can be configured on a servicebasis. The higher layer processors 1125 and 1130 process data generatedfrom a user service such as File Transfer Protocol (FTP) or Voice overInternet Protocol (VoIP) and provide the processed data to the MUX andDEMUX unit 1120. The higher layer processors 1125 and 1130 also processdata received from the MUX and DEMUX unit 1120 and provide the processeddata to a higher layer service application.

The controller 1110 checks a scheduling command, for example, a UL grantreceived through the transceiver 1105 and controls the transceiver 1105and the MUX and DEMUX unit 1120 to perform a UL transmission inappropriate transmission resources at an appropriate time point. Thecontroller 1110 also determines an operation to perform in an SCellsubframe based on the frequency bands and UL/DL configuration of a PCelland the SCell and controls the transceiver 1105 according to thedetermined operation.

FIG. 12 is a block diagram of an example embodiment of an eNB accordingto this disclosure.

Referring to FIG. 12, the eNB includes a transceiver 1205, a controller1210, a MUX and DEMUX unit 1220, a control message processor 1235, oneor more higher layer processors 1225 and 1230, and a scheduler 1215.

The transceiver 1205 transmits data and a specific control signal in aDL carrier and receives data and a specific control signal in a ULcarrier. If a plurality of carriers are configured, the transceiver 1205can transmit and receive data and a control signal in the plurality ofcarriers.

The MUX and DEMUX unit 1220 multiplexes data generated from the higherlayer processors 1225 and 1230 or the control message processor 1235, ordemultiplexes data received from the transceiver 1205 and provides thedemultiplexed data to the higher layer processors 1225 and 1230 or thecontroller 1210. The control message processor 1235 processes a controlmessage received from a UE and performs a necessary operation accordingto the processed control message, or generates a control message to betransmitted to the UE and provides the control message to a lower layer.

The higher layer processors 1225 and 1230 can be configured on a UEbasis and on a service basis. The higher layer processors 1225 and 1230process data generated from a user service such as FTP or VoIP andprovide the processed data to the MUX and DEMUX unit 1220. The higherlayer processors 1225 and 1230 also process data received from the MUXand DEMUX unit 1220 and provide the processed data to a higher layerservice application.

The controller 1210 determines SCell subframes in which the UE willreceive a DL signal and transmit a UL signal based on the frequencybands and UL/DL configurations of a PCell and the SCell and controls thetransceiver 1205 based on the determination.

The scheduler 1215 allocates transmission resources to the UE at anappropriate time point, taking into account the buffer state, channelstate, and active time of the UE so that the transceiver 1205 canprocess a signal received from the UE or a signal to be transmitted tothe UE.

The propagation environments of the PCell and the SCell can be differentgreatly depending on the frequency bands of the PCell and the SCell. Asa result, DL reception times can be different between the serving cells.Accordingly, SCell subframe n can overlap at least partially with PCellsubframe n and a PCell subframe adjacent to PCell subframe n on the timeaxis.

FIGS. 13A and 13B illustrate example embodiments of situations in whichsubframes of serving cells are overlapped with each other.

Referring to FIG. 13A, a reception time of an SCell precedes a receptiontime of a PCell. In this case, SCell subframe n (a subframe 1305) ispartially overlapped with PCell subframe n (a subframe 1315) and PCellsubframe n−1 (a subframe 1310). Referring to FIG. 13B, if the receptiontime of the SCell is later than the reception time of the PCell, SCellsubframe n (a subframe 1320) is partially overlapped with PCell subframen (a subframe 1325) and PCell subframe n+1 (a subframe 1330).

In the above cases, the UE should consider PCell subframe n and a PCellsubframe adjacent to PCell subframe n in determining an operation toperform in SCell subframe n.

FIG. 14 is a flowchart illustrating an example embodiment of anoperation of a UE in a subframe of an SCell according to thisdisclosure.

Referring to FIG. 14, the UE receives system information about a servingcell and acquires the UL/DL configuration of the serving cell from thesystem information in operation 1400. Then the UE starts an RRCconnection establishment procedure for the serving cell. Upon completionof the RRC connection establishment procedure, the serving cell becomesa PCell for the UE.

In operation 1405, the UE reports its capabilities to an eNB by acapability information message. The capability information message caninclude information about UE-supported band combinations. The UEadditionally reports information indicating availability of simultaneoustransmission and reception for an inter-TDD band combination.

In operation 1410, an SCell is configured for the UE. Specifically, theSCell is configured by transmitting an RRC control message includingSCell configuration information to the UE by the eNB and establishing asignal path to support the frequency band of the SCell based on theSCell configuration information by the UE, so that data transmission andreception can be performed in the SCell. The SCell configurationinformation includes, for example, information about a center frequencyof the SCell, radio resources allocated to the UE in the SCell, and theUL/DL configuration of the SCell.

In operation 1415, the UE waits until SCell subframe n starts in orderto determine an operation to perform in SCell subframe n.

The UE checks whether the following conditions are satisfied inoperation 1420. If the conditions are satisfied, the UE goes tooperation 1430 and otherwise, the UE goes to operation 1425.

-   -   The PCell and the SCell have different frequency bands        (inter-band TDD carrier aggregation).    -   The PCell and the SCell have different UL/DL configurations.

If the conditions are not satisfied, this means that the PCell and theSCell have the same frequency band or that although the PCell and theSCell have different frequency bands, they have the same UL/DLconfiguration. If the PCell and the SCell have the same frequency band,the same UL/DL configuration should be assigned to the PCell and theSCell.

In operation 1425, the UE determines the type of SCell subframe n basedon the UL/DL configuration of the SCell and determines an operation toperform according to the type of SCell subframe n. The UE operates asfollows according to the types of subframes.

-   -   If subframe n is a D subframe, the UE determines to receive a        PDCCH/PHICH/PDSCH in subframe n.    -   If subframe n is an S subframe, the UE determines to receive a        PDCCH/PHICH and when needed, to perform an UpPTS transmission in        subframe n.    -   If subframe n is a U subframe, the UE determines to transmit a        PUSCH and an SRS in subframe n, when needed.

The above operations can be limited by another solution or a requirementimposed on the UE. For example, if subframe n does not correspond to anactive time in a DRX operation or overlaps with a measurement gap beinga time period during which the UE suspends an uplink transmission and adownlink reception, the above operations can not be performed.

If the above conditions are satisfied, this implies that the PCell andthe S Cell have different frequency bands and different UL/DLconfigurations. In operation 1430, the UE determines whethersimultaneous transmission and reception is available in a current bandcombination (such as the combination of the frequency bands of the PCelland the SCell). Or the UE determines whether information indicatingsupport of simultaneous transmission and reception in the current bandcombination has been reported to the eNB. If the determination result isaffirmative, the UE goes to operation 1425 and otherwise, the UE goes tooperation 1435.

In operation 1435, the UE determines an operation to perform in SCellsubframe n of the SCell based on the type of a PCell subframe overlappedwith SCell subframe n. In general, the frame boundaries of the PCell andthe SCell are aligned and PCell subframe n is overlapped with SCellsubframe n. However, if the frame boundaries of the PCell and the SCellare not aligned, the UE operates as follows.

-   -   In the case where the DL reception time of the SCell is earlier        than the DL reception time of the PCell (that is, the start of        SCell subframe n is before the start of PCell subframe n) and        SCell subframe n is a U subframe, if at least one of PCell        subframe n−1 and PCell n is a D subframe, the eNB schedules the        UE not to perform a UL transmission in SCell subframe n. Even        though a UL transmission, for example, a non-adaptive        retransmission is scheduled in SCell subframe n, the UE        increases CURRENT_TX_NB by 1 without performing the UL        transmission.    -   In the case where the DL reception time of the SCell is later        than the DL reception time of the PCell (that is, the start of        SCell subframe n is behind the start of PCell subframe n of the        PCell) and SCell subframe n is a U subframe, if at least one of        PCell subframe n and PCell subframe n+1 is a D subframe, the eNB        schedules the UE not to perform a UL transmission in SCell        subframe n. Even though a UL transmission, for example, a        non-adaptive retransmission is scheduled in SCell subframe n,        the UE increases CURRENT_TX_NB by 1 without performing the UL        transmission.    -   In the case where the DL reception time of the SCell is earlier        than the DL reception time of the PCell (that is, the start of        SCell subframe n is before the start of PCell subframe n) and        SCell subframe n is a D subframe, if at least one of PCell        subframe n−1 and PCell subframe n is a U subframe, the eNB does        not schedule a DL transmission in SCell subframe n for the UE.        The UE does not perform a DL reception in SCell subframe n. That        is, the UE does not receive a PDCCH/PHICH/PDSCH in SCell        subframe n. Only when SCell subframe n is a D subframe and both        PCell subframe n−1 and PCell subframe n are D subframes (or        either of PCell subframe n−1 and PCell subframe n is not a U        subframe), the UE receives a DL signal in SCell subframe n.    -   In the case where the DL reception time of the SCell is later        than the DL reception time of the PCell (that is, the start of        SCell subframe n is behind the start of PCell subframe n) and        SCell subframe n is a D subframe, if at least one of PCell        subframe n and PCell subframe n+1 is a U subframe, the eNB does        not schedule a DL transmission in SCell subframe n for the UE.        The UE does not perform a DL reception in SCell subframe n. That        is, the UE does not receive a PDCCH/PHICH/PDSCH in SCell        subframe n. Only when SCell subframe n is a D subframe and both        PCell subframe n, and PCell subframe n+1 are D subframes or        PCell subframe and PCell subframe n+1 are a D subframe and an S        subframe, respectively (or either of PCell subframe n and PCell        subframe n+1 is not a U subframe), the UE receives a DL signal        in SCell subframe n.

In other words, if any of PCell subframes overlapped at least partiallywith SCell subframe n is a U subframe, the UE is not allowed to receivea DL signal in SCell subframe n and the eNB schedules the UEaccordingly.

If any of PCell subframes overlapped at least partially with SCellsubframe n is a D subframe, the UE is not allowed to transmit a ULsignal in SCell subframe n and the eNB schedules the UE accordingly.

The UE and the eNB that operate according to the procedure of FIG. 14have the configurations illustrated in FIGS. 11 and 12, respectively.

To support inter-frequency handover, a UE is configured to measure afrequency other than a current serving frequency (such as a servingcell) from time to time. The UE is not allowed to transmit data orreceive data in the serving frequency according to the hardware of theUE while the UE is measuring the non-serving frequency. For example, ifthe UE includes a single RF front end, the UE can not transmit orreceive a signal in the serving frequency during measurement of thenon-serving frequency using the single RF front end.

To prevent performance degradation caused by data transmission to the UEor UL scheduling for the UE by an eNB during measurement of the UE, atime period during which the UE suspends an uplink transmission and adownlink reception can be preset for the UE. This time period isreferred to as a measurement gap. The UE does not transmit data and tuneits receiver to the carrier frequencies of a PCell and an SCell duringthe measurement gap.

The measurement gap can start at the start of a specific subframe andlast a predetermined time, for example, 6 ms. The subframe in which themeasurement gap starts is specified by a parameter, gapOffset. The eNBtransmits measurement gap configuration information to the UE,simultaneously with indicating measurement of non-serving frequencies.For example, the measurement gap configuration information can includethe following information.

gapOffset: information indicating a subframe in which a measurement gapstarts.

Measurement Gap Repetition Period (MGRP) information: informationindicating whether an MGRP is 40 ms or 80 ms.

The UE determines a time period corresponding to the measurement gapbased on the measurement gap configuration information and performs noneof DL reception and UL transmission in the serving cells during themeasurement gap.

FIG. 15 illustrates an example embodiment of a configuration of ameasurement gap according to this disclosure.

Referring to FIG. 15, gapOffset specifies subframe 3 (a subframe 1505)of a radio frame with SFN n. A first measurement gap 1510 starts at thestarting of subframe 3 and lasts 6 ms. A second measurement gap 1520starts in the same subframe of radio frame n+m, that is, subframe 3 ofradio frame n+m after a predetermined time period from the starting ofthe first measurement gap 1510. Herein, m is an integer determined by anMGRP.

Even though a UE completely measures non-serving frequencies during onemeasurement gap, the measurement gap repeatedly occurs until an eNBreleases the measurement gap.

If only one serving cell is configured for the UE, the start of ameasurement gap can be determined accurately simply by specifying astarting subframe. However, if a plurality of serving cells areconfigured for the UE, the start of a measurement gap can not bedetermined accurately simply by specifying a starting subframe.

FIG. 16 illustrates an example embodiment of a problem involved inconfiguring a measurement gap, when the subframe boundaries of servingcells are not aligned.

Referring to FIG. 16, the DL reception time of serving cell 1 is laterthan the DL reception time of serving cell 2 and gapOffset specifies thestart of a measurement gap as subframe n. With respect to subframe n (asubframe 1610) of serving cell 1, a measurement gap 1620 lasts fromsubframe n of serving cell 1 to subframe n+5 of serving cell 1. Astarting part of subframe n (a subframe 1615) of serving cell 2 is notincluded in the measurement gap 1620, whereas a starting part ofsubframe n+6 (a subframe 1630) of serving cell 2 is included in themeasurement gap 1620.

With respect to subframe n of serving cell 2, a measurement gap 1625lasts from subframe n of serving cell 2 to subframe n+5 of serving cell2. A last part of subframe n−1 (a subframe 1605) of serving cell 1 isincluded in the measurement gap 1625, whereas a last part of subframen+5 of serving cell 1 is not included in the measurement gap 1625.

A measurement gap is a time period during which a UE suspends an uplinktransmission and a downlink reception and can measure non-servingfrequencies, and an eNB agrees not to schedule the UE in the measurementgap. If the UE and the eNB have different measurement gaps, the eNB cantransmit and receive data in a subframe in which the UE does nottransmit and receive data. For example, if the eNB uses the measurementgap 1620 configured with respect to serving cell 1 and the UE uses themeasurement gap 1625 configured with respect to serving cell 2, themeasurement gap 1625 of the UE starts before subframe n−1 of servingcell 1 and thus the UE does not transmit and receive data in subframen−1 of serving cell 1. However, since the measurement gap 1620 of theeNB does not include subframe n−1 of serving cell 1, the eNB cantransmit data to the UE in subframe n−1 of serving cell 1.

To avert the above problem, the UE and the eNB determine the start of ameasurement gap based on the same serving cell. In an embodiment of thepresent disclosure, a PCell can be used as a reference for a measurementgap. In the illustrated case of FIG. 16, if serving cell 1 is a PCell,the measurement gap 1620 is used and if serving cell 2 is a PCell, themeasurement gap 1625 is used.

FIG. 17 illustrates configuration of a measurement gap, when the DLreception time of an SCell is earlier than the DL reception time of aPCell. In FIG. 17, both an eNB and a UE set the same measurement gap1720 starting in PCell subframe n.

Referring to FIG. 17, SCell subframe n (a subframe 1715) is earlier thanPCell subframe n (a subframe 1710). Therefore, SCell subframe n isoverlapped with PCell subframe n−1 (a subframe 1705) over a specificpart 1725. The measurement gap 1720 is configured to start at the startof PCell subframe n and end at the end of PCell subframe n+5. If theSCell precedes the PCell, a starting part 1735 of SCell subframe n+6 (asubframe 1730) is included in the measurement gap 1720 and the eNB doesnot transmit data to the UE in SCell subframe n+6. While it isdetermined by the subframe index of SCell subframe n+6 that SCellsubframe n+6 does not belong to the measurement gap 1720, the startingpart 1735 of SCell subframe n+6 is included in the measurement gap 1720in the actual time domain. Thus, the UE does not perform datatransmission and reception in SCell subframe n+6, determining that theeNB will not schedule the UE in SCell subframe n+6.

If the SCell precedes the PCell as described above, a starting part of aspecific SCell subframe is included in a measurement gap and thus theeNB does not transmit data to the UE in the SCell subframe. Although theSCell subframe is determined not to be included in the measurement gapbecause of its subframe index, the starting part of the SCell subframeis included in the measurement gap. Therefore, the UE does not transmitand receive data in the SCell subframe, determining that the eNB willnot schedule the UE in the SCell subframe.

FIG. 18 illustrates an example embodiment of a configuration of ameasurement gap, when the DL reception time of a PCell is earlier thanthe DL reception time of an SCell. In FIG. 18, both an eNB and a UE setthe same measurement gap 1820 starting in PCell subframe n.

Referring to FIG. 18, PCell subframe n (a subframe 1805) is overlappedwith SCell subframe n−1 (a subframe 1815) over a specific part 1830. IfSCell subframe n is behind PCell subframe n, a last part 1830 of SCellsubframe n−1 (a subframe 1815) is included in the measurement gap 1820and thus the eNB does not transmit data to the UE in SCell subframe n−1.While it is determined by the subframe index of SCell subframe n−1 thatSCell subframe n−1 does not belong to the measurement gap 1820, the lastpart 1830 of SCell subframe n−1 is included in the measurement gap 1820in the time domain. Thus, the UE does not perform data transmission andreception in SCell subframe n−1, determining that the eNB will notschedule the UE in SCell subframe n−1.

FIG. 19 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure.

Referring to FIG. 19, the UE receives system information about a servingcell, acquires information required for an RRC connection establishmentprocedure, and starts the RRC connection establishment procedure for theserving cell in operation 1900. Once the RRC connection establishmentprocedure is completed, the serving cell becomes a PCell for the UE.

In operation 1905, the UE reports its capabilities to an eNB by a UEcapability information message. If the UE supports CA, the UE capabilityinformation message includes information about UE-supported bandcombinations and the eNB determines what cell having what specificfrequency to configure as an SCell based on the supported bandcombination information.

In operation 1910, the SCell is configured for the UE. The SCell isconfigured by transmitting an RRC control message including SCellconfiguration information from the eNB to the UE and establishing asignal path for supporting the frequency band of the SCell based on theSCell configuration information by the UE, or setting a transceiver ofthe UE to transmit and receive data in the SCell. The SCellconfiguration information includes, for example, information about acenter frequency of the SCell, radio resources allocated to the UE inthe SCell, and the like.

In operation 1915, the eNB configures a measurement gap for the UE. Themeasurement gap is configured by allocating gapOffset to, the UE. Forexample, gapOffset is classified into a first gapOffset ranging from 0to 39 and a second gapOffset ranging from 0 to 79 and one of the firstand second gapOffsets is used. An MGRP corresponding to the firstgapOffset is 40 ms and an MGRP corresponding to the second gapOffset is80 ms.

The SCell configuration (operation 1910) and the measurement gapconfiguration (operation 1915) can take place simultaneously by one RRCmessage or sequentially by separate control messages. In the lattercase, the order of the SCell configuration and the measurement gapconfiguration can be changed.

In operation 1920, the UE determines an SFN and a subframe number thatspecify the starting point of the measurement gap by a predeterminedequations and a gap offset.

In an embodiment of the present disclosure, the SFN of a radio frame inwhich the measurement gap starts can be calculated by equation (1).SFN mod T=FLOOR(gapOffset/10)  (1)

where T=MGRP/10.

In an embodiment of the present disclosure, the number of a subframe inwhich the measurement gap starts can be calculated by equation (2).subframe=gapOffset mod 10  (2)

In operation 1925, the UE determines the start of the measurement gap byapplying the SFN and the subframe number specifying the start of themeasurement gap to the PCell. For example, if subframe number n iscalculated by equation (2), the UE determines that the measurement gapstarts in PCell subframe n.

In operation 1930, the UE determines in what subframe of what servingcell to discontinue transmission and reception based on the determinedmeasurement gap. The determination can be made as follows.

-   -   The UE does not perform either UL transmission or DL reception        in subframe n to subframe n+5 of the PCell and subframe n to        subframe n+5 of an active SCell.    -   The UE does not perform (or discontinues) DL reception in a        subframe immediately before the measurement gap in SCell p        having a DL reception time earlier than the DL reception time of        the PCell, that is, in subframe n−1 of SCell p from among active        SCells. In other words, DL reception is performed in the other        subframes except for subframe n−1 to subframe n+5 of SCell p,        unless otherwise prohibited.    -   The UE does not perform (or discontinues) DL reception in a        subframe immediately after the measurement gap in SCell k having        a DL reception time later than the DL reception time of the        PCell, that is, in subframe n+6 of SCell k from among active        SCells. In other words, DL reception is performed in the other        subframes except for subframe n to subframe n+6 of SCell k,        unless otherwise prohibited.    -   The UE does not perform (or discontinues) UL transmission in a        subframe immediately after the measurement gap in the PCell and        all active SCells, that is, in subframes n+6 of the PCell and        all active SCells. In other words, UL transmission is performed        in the other subframes except for subframes n to subframes n+6        of the PCell and the SCells, if the UL transmission is not        prohibited otherwise and the UL transmission is scheduled.        Because UL subframe n+6 is overlapped with DL subframe n+5 on        the time axis due to a Timing Advance (TA) applied to an OFDM        system, UL transmission is not performed in subframe n+6.

In operation 1935, the UE discontinues (or blocks) DL reception in aspecific subframe of a specific serving cell and UL transmission in aspecific subframe of a specific serving cell, according to thedetermination.

Discontinuation of DL reception in a subframe means that the UE does notmonitor a PDCCH and buffer a PDSCH in the subframe, and even though theUE is scheduled to receive a DL HARQ feedback in the subframe, the UEdoes not receive the HARQ feedback in the subframe. The non-receivedHARQ feedback is a response to a PUSCH transmission. If a PUSCH istransmitted and an HARQ feedback is not received for the PUSCH, the UEperforms an HARQ operation, determining that the HARQ feedback is anACK. If a PUSCH is not transmitted and an HARQ feedback is not received,the UE performs an HARQ operation, determining that the HARQ feedback isa NACK.

Discontinuation of UL transmission in a subframe means that even thoughPUCCH transmission or PUSCH transmission is scheduled in the subframe,the UE does not perform the UL transmission. Particularly when the UEdoes not transmit a PUSCH, the UE increases CURRENT_TX_NB by 1 andmaintains CURRENT_IRV at a current value.

Since the subframe immediately before the measurement gap can bepartially overlapped with the measurement gap in the time domain, DLreception is not performed in the subframe immediately before themeasurement gap in the embodiment of the present disclosure illustratedin FIG. 9. However, if the overlapped period is very short relative to asubframe length in the time domain, the probability of receiving a DLsignal in the subframe and succeeding in decoding the DL signal at theUE can be very high. An embodiment of receiving a DL signal afterpuncturing, instead of quitting DL reception in a subframe overlappedwith a measurement gap in the time domain will be described below.

One subframe includes 14 OFDM symbols. A PDCCH is transmitted in thefirst n symbols of the subframe and a PDSCH is transmitted in the last(14−n) symbols. Even though the PDSCH is partially lost, there is apossibility that the PDSCH is decoded successfully using a received partof the PDSCH. In contrast, if even a part of the PDCCH is not received,the PDCCH may not be interpreted correctly. Therefore, a DL subframe tobe received should be confined to a subframe having a last partoverlapped with a measurement gap. That is, even though a subframeimmediately before the measurement gap, that is, subframe n−1 ispartially overlapped with the measurement gap, DL reception is possiblein subframe n−1. Thus, DL reception may be allowed in subframe n−1. Inaddition, the start of the measurement gap is appropriately adjusted soas to prevent a subframe immediately after the measurement gap fromoverlap with the measurement gap on the time axis.

FIG. 20 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. Operations 2000, 2005, 2010, 2015, and 2020 are performed inthe same manner as operations 1900, 1905, 1910, 1915, and 1920illustrated in FIG. 19 and thus will not be described herein.

Referring to FIG. 20, the UE determines whether there is any SCellhaving a DL reception time earlier than the DL reception time of a PCellamong active SCells in operation 2023. In the absence of an SCell havinga DL reception time earlier than the DL reception time of the PCell,that is, if the DL reception time of the PCell is earlier than the DLreception times of all active SCells, the UE proceeds to operation 2025.In the presence of at least one SCell having a DL reception time earlierthan the DL reception time of the PCell, the UE goes to operation 2045.

If the UE goes to operation 2025, this means that if a measurement gapstarts in PCell subframe n, the measurement gap is overlapped with SCellsubframe n−1 in the time domain but is not overlapped with SCellsubframe n+6 in the time domain. Therefore, the UE determines PCellsubframe n to be the starting subframe of the measurement gap inoperation 2025 and determines a subframe in which data transmission andreception is to be discontinued and a subframe in which only datatransmission is to be discontinued in the same manner as in operation1930 illustrated in FIG. 19, in operation 2030. That is, the UEdetermines to discontinue data transmission and reception in subframes nto subframes n+5 of the PCell and an SCell and determines to discontinuedata transmission in subframes n+6 of the PCell and the SCell. That is,the UE discontinues data transmission in subframes n+6 of the PCell andthe SCell and tunes its receiver to one of the carrier frequencies ofthe PCell and the SCell. An eNB does not schedule data transmission inthe subframes in which the UE discontinues data transmission andreception and does not monitor reception of a DL signal in thesubframes.

In operation 2035, the UE receives a DL signal in subframes n−1 of theactive SCells. For example, in FIG. 18, the UE discontinues reception inthe part 1830 over which the measurement gap is overlapped with subframen−1 of the SCell in the time domain.

In operation 2040, the UE discontinues DL reception in a specificsubframe of a specific serving cell and UL transmission in a specificsubframe of a specific serving cell according to the determination madein operation 2030.

If the UE goes from operation 2023 to operation 2045, this means that ifthe start of the measurement gap is set to PCell subframe n, themeasurement gap is not overlapped with SCell subframe n−1 in the timedomain but is overlapped with SCell subframe n+6 in the time domain.Therefore, the UE advances the star of the measurement gap from thestart of PCell subframe n by a predetermined time period so that a partoverlapped with the measurement gap is included in SCell subframe n−1.The predetermined time period corresponds to the difference between theearliest of the DL reception times of the active SCells and the DLreception time of the PCell. Consequently, the start of the measurementgap is determined to be subframe n of an SCell having the earliest DLreception time.

For example, in FIG. 17, the start of the measurement gap advances fromthe start of subframe n of the PCell by the predetermined time period1725. With the adjustment of the start of the measurement gap, a part ofsubframe n−1 of the PCell is overlapped with the measurement gap in thetime domain. However, the measurement gap is not overlapped withsubframe n+6 of any serving cell in the time domain. After determiningthe start of the measurement gap, the UE proceeds to operation 2050.Operation 2050 is performed in the same manner as operation 2030.

In operation 2055, the UE performs DL reception in PCell subframe n−1,while the UE discontinues reception in a time-domain part overlappedbetween PCell subframe n−1 and the measurement gap.

Operation 2060 is performed in the same manner as operation 2040.

In an embodiment of the present disclosure described later, the start ofa measurement gap is determined with respect to a serving cell having anearliest DL reception time, instead of a PCell.

FIG. 21 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. Operations 2100, 2105, 2110, 2115, and 2120 are performed inthe same manner as operations 1900, 1905, 1910, 1915, and 1920 and thuswill not be described herein.

Referring to FIG. 21, the UE determines the start of a measurement gapby applying an SFN and a subframe number that specify the start of themeasurement gap to a reference serving cell in operation 2125. Ifsubframe n is determined by equations (1) and (2), the UE determines themeasurement gap to start in subframe n of the reference serving cell.Herein, the reference serving cell is a serving cell having the earliestDL reception time from among current active serving cells. The referenceserving cell can be a PCell or an SCell. In the situation of FIG. 17,the reference serving cell is the PCell, while in the situation of FIG.18, the reference serving cell is the SCell.

In operation 2130, the UE determines in what subframe of what servingcell to discontinue transmission and reception according to thedetermined measurement gap. The determination is made as follows.

-   -   None of UL transmission and DL reception is performed (or both        UL transmission and DL reception are discontinued) in subframes        n to subframes n+5 of the PCell and an active SCell.    -   DL reception is not performed (or is discontinued) in subframes        immediately before the measurement gap, that is, subframes n−1        of the remaining serving cells except for the reference serving        cell. In another embodiment of the present disclosure, DL        reception is performed in time-domain parts not overlapped        between subframes n−1 and the measurement gap and DL reception        is not performed only in overlapped parts.    -   DL transmission is not performed (or is discontinued) in        subframes of the PCell and an active SCell, immediately after        the measurement gap, that is, subframes n+6 of the PCell and the        active SCell. In other words, if UL transmission is not        prohibited otherwise and is scheduled in the other subframes        except for subframes n to n+6 of the PCell and the SCell, UL        transmission is performed. The reason for not performing UL        transmission in subframe n+6 is that UL subframe n+6 is        overlapped with DL subframe n+5 on the time axis due to a TA        applied in the OFDM communication system.

In operation 2135, the UE discontinues DL reception in a specificsubframe of a specific serving cell and UL transmission in a specificsubframe of a specific serving cell according to the determination.

The UE and the eNB that operate in the procedures of FIGS. 19 and 20have the configurations illustrated in FIGS. 11 and 12, respectively.

In an OFDM communication system, an eNB should receive a signal from aUE before the end of a predetermined time period called a CP. To enableUL signals of UEs at different locations to arrive at the eNB before theend of the predetermined time period, the UL transmission times of theUEs should be adjusted. In an LTE system, a UE advances a ULtransmission time from a DL reception time by a TA. An eNB can determinea TA for each UE. Accordingly, a measurement gap configured with respectto a DL subframe is not aligned with a UL subframe and a subframeimmediately after the measurement gap is always overlapped with themeasurement gap in the TA. To guarantee the measurement gap, the UE doesnot perform UL transmission in a UL subframe immediately after themeasurement gap.

FIG. 22 illustrates an example embodiment of a relationship between ameasurement gap and a UL subframe in an FDD system. For the convenienceof description, subframe [before] represents a subframe immediatelybefore the measurement gap and subframe [after] represents a subframeimmediately after the measurement gap. In FIG. 22, a measurement gap2220 lasts from subframe n to subframe n+5 and subframe [before] andsubframe [after] are subframe n−1 and subframe n+6, respectively.

Referring to FIG. 22, UL subframe n is earlier than DL subframe n by aTA 2225 and thus UL subframe n+6 is overlapped with the measurement gap2220 by a TA 2235.

In a TDD system, even though subframe [after] is a UL subframe, it isnot always overlapped with a measurement gap because a DL and a ULcoexist in the same frequency area.

FIG. 23 illustrates an example embodiment of a timing adjustment of a ULsubframe in a TDD system.

Referring to FIG. 23, DL-to-UL switching (hereinafter, referred to asD-to-U switching) 2305 occurs during the time period of an S subframe2310 in the TDD system. The reception time of a U subframe is adjustedin the S subframe 2310 by a TA. Therefore, the length of the S subframe2310 is not 1 ms but (1−TA) ms. For example, if the TA is 0.05 ms, the Ssubframe 2310 is 0.995 ms long. When the UL switches to the DL, thesubframe timing is delayed by the TA. That is, an empty time period ofthe TA size occurs during U-to-D switching 2315.

In other words, the subframe timing is advanced by the TA during D-to-Uswitching and is delayed by the TA during U-to-D switching.

If more D-to-U switchings occur than U-to-D switchings in themeasurement gap, the measurement gap includes a part of subframe [after]and the UE does not perform UL transmission in subframe [after]. If thenumber of D-to-U switchings is equal to the number of U-to-D switchingsin the measurement gap, the measurement gap does not include subframe[after] and thus the UE can perform UL transmission in subframe [after].

Since subframes are arranged in a cyclic order of D, S, U, and D, onlywhen subframe [before] is a D subframe, subframe [after] is a U subframeand the number of D-to-U switchings is larger than the number of U-to-Dswitchings in the measurement gap. Therefore, the UE refers to the typeof subframe [before]. If subframe [before] is a D subframe and subframe[after] is a U subframe, the UE does not perform UL transmission insubframe [after]. If subframe [before] is not a D subframe and subframe[after] is a U subframe, the UE performs UL transmission in subframe[after], when needed.

If one serving cell exists or serving cells have the same UL/DLconfiguration, determination as to whether to perform UL transmission insubframe [after] based on the type of subframe [before] is effective.However, if the serving cells have different UL/DL configurations, aplurality of subframes [before] should be referred to and thus this rulemay not be applied.

The UE and the eNB should have the same knowledge of UL transmission orUL non-transmission in subframe [after] to thereby prevent waste oftransmission resources or battery consumption of the UE.

FIG. 24 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. If at least one of a plurality of subframes [before] is a Dsubframe, a UE does not perform UL transmission in subframe [after]being a U subframe.

Referring to FIG. 24, the UE recognizes that a measurement gap will endin subframe m in operation 2405.

In operation 2410, the UE determines whether a current system is an FDDsystem or a TDD system. If the current system is an FDD system, the UEgoes to operation 2415 and if the current system is a TDD system, the UEgoes to operation 2420. In operation 2415, the UE determines that ULtransmission is not allowed in U subframe m+1 and thus even though ULtransmission is scheduled in U subframe m+1, the UE controls itstransmitter to adjust related variables without UL transmission in Usubframe m+1. The related variables can include CURRENT_TX_NB,CURRENT_IRV, HARQ_FEEDBACK, and the like.

In operation 2420, the UE determines whether subframe m+1 is a Usubframe. If a plurality of serving cells are configured for the UE, theUE determines at least one of subframes m+1 of the serving cells is a Usubframe. If at least one of subframes m+1 of the serving cells is a Usubframe, the UE goes to operation 2430 and if none of subframes m+1 ofthe serving cells are U subframes, the UE goes to operation 2425. Inoperation 2425, the UE performs an appropriate operation according tothe type of subframe m+1. That is, if subframe m+1 is a D subframe, theUE performs a DL operation such as PDCCH reception. If subframe m+1 isan S subframe, the UE performs an operation corresponding to the Ssubframe.

In operation 2430, the UE determines whether only one serving cell hasbeen configured (or only one serving cell is active), or a plurality ofserving cells have been configured and active serving cells have thesame UL/DL configuration. If only one serving cell has been configuredor all serving cells have the same UL/DL configuration, the UEdetermines whether subframe [before] is a D subframe in 2435. Ifsubframe [before] is a D subframe, the UE goes to operation 2415 and ifsubframe [before] is not a D subframe, the UE goes to operation 2445. Onthe other hand, if a plurality of serving cells have been configured andhave different UL/DL configurations, the UE goes to operation 2440.

In operation 2440, the UE determines whether at least one of subframes[before] of the configured serving cells (or the active service cells)is a D subframe. If at least one of subframes [before] of the configuredserving cells (or the active service cells) is a D subframe, the UE doesnot perform UL transmission in a U subframe from among subframes m+1 inoperation 2415. Herein, the UE performs a normal operation in subframesm+1 that are not U subframes. In operation 2445, the UE performs ULtransmission in a U subframe from among subframes m+1 if the U subframeis scheduled for UL transmission and the UL transmission is notprohibited otherwise. As in operation 2415, the UE performs a normaloperation in subframes m+1 which are not U subframes.

Even though subframe [after] is a U subframe, an HD UE is not allowed toperform UL transmission if subframe [after] of the PCell is not a Usubframe.

FIG. 25 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. Operations 2505, 2510, 2515, 2520, and 2525 are performed inthe same manner as operations 2405, 2410, 2415, 2420, and 2425illustrated in FIG. 24 and will not be described herein.

Referring to FIG. 25, in operation 2530, the UE determines whether onlyone serving cell has been configured (or only one serving cell isactive), or a plurality of serving cells have been configured and activeserving cells have the same UL/DL configuration. If only one servingcell has been configured or all serving cells have the same UL/DLconfiguration, the UE performs the same operation as operation 2435 ofFIG. 24, in operation 2535. If a plurality of serving cells have beenconfigured and have different UL/DL configurations, the UE goes tooperation 2533.

In operation 2533, the UE determines whether simultaneous transmissionand reception is available in a current band combination (such aswhether the UE can operate in FD). Or the UE determines whetherinformation indicating support of simultaneous transmission andreception in the current band combination has been transmitted to aneNB. If the UE can operate in FD, the UE performs the same operation asoperation 2440 of FIG. 24 in operation 2540. If the UE does not operatein FD, that is, the UE operates in HD, the UE proceeds to operation2537.

In operation 2537, the UE determines whether subframe [before] of aPCell is a D subframe. If PCell subframe [before] is a D subframe, theUE does not perform UL transmission in a U subframe from among subframesm+1 in operation 2515. Herein, the UE performs a normal operation insubframes m+1 that are not U subframes. If PCell subframe [before] isnot a D subframe, the UE performs UL transmission in a U subframe fromamong subframes m+1 if the U subframe is scheduled for UL transmissionand the UL transmission is not prohibited otherwise, in operation 2545.As in operation 2515, the UE performs a normal operation in subframesm+1 which are not U subframes.

FIG. 26 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. FIG. 26 illustrates another operation of an HD UE.Operations 2605, 2610, 2615, 2620, 2625, 2630, 2635, 2633, 2640, and2645 are performed in the same manner as operations 2505, 2510, 2515,2520, 2525, 2530, 2535, 2533, 2540, and 2545 of FIG. 25 and thus willnot be described herein.

Referring to FIG. 26, the UE determines whether subframe [before] of aPCell is not a D subframe and subframe [after] of the PCell is a Usubframe in operation 2637. If the two conditions are satisfied, the UEcan perform UL transmission in subframe [after] of another serving celland thus the UE proceeds to operation 2645. If at least one of the twoconditions is not satisfied, the UE does not perform UL transmission insubframes m+1 of all serving cells in operation 2615. In operation 2645,the UE performs UL transmission in subframe m+1 which is a U subframefrom among subframes m+1 of the PCell and SCells, if the UL transmissionis scheduled in the subframe and is not prohibited otherwise.

To apply different UL/DL configurations to serving cells, the servingcells should have different frequency bands, as described before. Inview of the structure of a UE, the UE should use different RF circuitsfor different frequency bands. In this case, it is possible to controlan operation in subframe [after] on a serving cell basis. That is, onlysubframe [before] of serving cell x is considered in determining anoperation in subframe [after] of serving cell x. In this case, while ameasurement gap applies to a different time period in each serving cell,6 ms can be ensured for the measurement gap.

FIG. 27 illustrates an example embodiment of serving cells havingdifferent UL/DL configurations.

Referring to FIG. 27, UL/DL configuration 1 and UL configuration 3 ofTABLE 1 are assigned to serving cell 1 and serving cell 2, respectivelyand a measurement gap lasts from subframe 7 to subframe 2. A UE usesdifferent RF front ends for serving cell 1 and serving cell 2. Let theRF front end applied to serving cell 1 be denoted by RF1 and the RFfront end applied to serving cell 2 be denoted by RF2. Then the UEmeasures one or more non-serving frequencies using one of the RF endsRF1 and RF2. If the UE measures the non-serving frequencies using the RFend RF1, since subframe [before] of serving cell 1 is not a D subframe,a 6-ms period 2705 is given to the RF end RF1 despite UL transmission insubframe [after] being subframe 3. If the UE measures the non-servingfrequencies using the RF end RF2, a 6-ms period 2710 is not ensured dueto UL transmission in subframe U because subframe [before] is a Dsubframe and subframe [after] is a U subframe. Therefore, the UE doesnot perform UL transmission. Therefore, even though an eNB does not haveknowledge of an RF end used for measurement at the UE, the eNB candetermine clearly whether subframe [after] is available at least at aserving cell level.

FIG. 28 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. Operations 2805, 2810, 2815, 2820, 2825, 2830, 2835, and2845 are performed in the same manner as operations 2405, 2410, 2415,2420, 2425, 2430, 2435, and 2445 illustrated in FIG. 24.

Referring to FIG. 28, if the UE proceeds to operation 2840, this impliesthat a plurality of serving cells have been configured for the UE, theserving cells have different UL/DL configurations, and at least onesubframe m+1 of the serving cells is a U subframe. The UE checkssubframes [before] and subframes [after] of active serving cells amongthe configured serving cells. For a serving cell having a D subframe assubframe [before] and a U subframe as subframe [after], the UE goes tooperation 2850 and for the other cells, the UE goes to operation 2855.If the UE goes to operation 2850, this means that if the UE performs ULtransmission in subframe [m+1] of the corresponding serving cell, a 6-msperiod can not be guaranteed. Even though UL transmission is scheduledin subframe m+1 of the serving cell, the UE just adjusts relatedvariables without performing the UL transmission.

In operation 2855, the UE determines whether the UE operates in FD or HDin a current configuration or a current band combination. If the UEoperates in HD, the UE goes to operation 2860 and if the UE operates inFD, the UE goes to operation 2865.

In operation 2860, the UE determines an operation to perform in subframe[after] of an SCell based on the type of a PCell subframe overlappedwith subframe [after] of the SCell (such as the type of subframe [after]of the PCell, the type of subframe [after] and subframe [after+1] of thePCell, or the type of subframe [after−1] of the PCell).

In operation 2865, the UE performs an operation to perform in subframe[after] of the SCell based on the type of subframe [after] of the SCell.

In the example of FIG. 27, subframe [before] of a serving cell that is areference for the start of a measurement gap affects an operation insubframe [after] of the serving cell. For example, if the measurementgap starts in subframe 7 of serving cell 1, UL transmission is possiblein subframe [after] of serving cell 2 having a D subframe as subframe[before] as well as in subframe [after] of serving cell 1 becausesubframe [before] of serving cell 1 is not a D subframe. If themeasurement gap starts in subframe 7 of serving cell 2, UL transmissionis not allowed in subframe [after] of serving cell 1 having subframe[before] which is not a D subframe as well as in subframe [after] ofserving cell 2 because subframe [before] of serving cell 2 is a Dsubframe.

Therefore, if the eNB and the UE have knowledge of a serving cell havinga subframe boundary or a subframe in which the measurement gap start, anoperation to perform in subframe [after] of every serving cell can bedetermined based on subframe [before] of the serving cell.

FIG. 29 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. Operations 2900, 2905, 2910, 2915, and 2920 are performed inthe same manner as operations 1900, 1905, 1910, 1915, and 1920illustrated in FIG. 19.

Referring to FIG. 29, a measurement gap starts at the start of asubframe indicated by an SFN and a subframe number that specify thestart of the measurement gap from among subframes of a predeterminedreference cell in operation 2925. In an embodiment of the presentdisclosure, the reference cell can be a PCell. In another embodiment ofthe present disclosure, the reference cell can be a serving cell havingthe earliest DL reception time among current active serving cells. Inanother embodiment of the present disclosure, the reference cell can bea serving cell having subframe [before] that is not a D subframe. Inanother embodiment of the present disclosure, the reference cell can bea serving cell having the last UL reception time among current activeserving cells.

In operation 2930, the UE determines whether subframe [before] of thereference cell, for example, the PCell is a D subframe and at least oneof current active serving cells including the reference cell hassubframe [after] being a U subframe. If the two conditions aresatisfied, the UE goes to operation 2935 and if at least one of theconditions is not satisfied, the UE goes to operation 2940.

In operation 2935, the UE does not perform UL transmission in subframe[after] being a U subframe from among subframes [after] of the servingcells. In subframes [after] of the serving cells which are not Usubframes, the UE performs a normal operation. For example, if the UEoperates in HD, the UE performs an operation in subframe [after] of anSCell based on the types of subframe [after] and its adjacent subframeof the PCell. If the UE operates in FD, the UE performs an operation insubframe [after] of a serving cell based on the type of subframe [after]of the serving cell.

In operation 2940, the UE determines whether it operates in FD or HD ina current configuration or a current band combination. If the UEoperates in HD, the UE goes to operation 2945 and if the UE operates inFD, the UE goes to operation 2950. In operation 2945, the UE determinesan operation to perform in subframe [after] of an SCell based on thetype of a PCell subframe overlapped with subframe [after] of the SCell(such as the type of subframe [after] of the PCell, the types ofsubframes [after] and [after+1] of the PCell, or the type of subframe[after−1] of the PCell). In operation 2950, the UE determines anoperation to perform in subframe [after] of the SCell based on the typeof subframe [after] of the SCell.

The UE and the eNB that operate according to the procedures of FIGS. 28and 29 have the configurations illustrated in FIGS. 11 and 12,respectively.

An embodiment of the present disclosure described below provides amethod and apparatus for starting a measurement gap based on a servingcell having the last of the DL reception times of current active servingcells and performing no UL transmission in subframe [after] that is a Usubframe. This is because the type of a subframe of a serving cell thatprovides a reference for the start of the measurement gap affects anoperation in subframe [after]. The influence on subframe [before] isminimized by setting the subframe boundary of a hindmost serving cell asthe reference for the start of the measurement gap.

FIG. 30 illustrates an example embodiment of subframe sets according toan embodiment of the present disclosure. In FIG. 30, the starts andlengths of subframe sets are illustrated according to the types ofsubframes [before] and [after] of the subframe sets. A subframe setrefers to a plurality of consecutive subframes. For the convenience ofdescription, the subframe set includes subframes n, n+1, n+2, n+3, n+4,and n+5.

Referring to FIG. 30, when a measurement gap starts at the start of a6-ms subframe set (referred to as a medium set), the measurement gapdoes not affect subframe [after] of the same serving cell. When themeasurement gap starts at the start of a subframe set shorter than 6 ms(referred to as a short set), a part of subframe [after] of the sameserving cell is overlapped with the measurement gap and thus a UE is notallowed to perform transmission and reception in subframe [after]. Whenthe measurement gap starts at the start of a subframe set longer than 6ms (referred to as a long set), the measurement gap does not affectsubframe [after] of the same serving cell.

Nine combinations of subframes [before] and [after] can be producedaccording to the types of subframes [before] and [after], as follows.

TABLE 7 Subframe Subframe [before] Subframe [after] Subframe [before][after] D D U S D U S D D S S U U D S S U U

It is not necessary to consider a case where both subframes [before] and[after] are S subframes because the case does not exist from theperspective of UL/DL configurations. If subframe [before] is a Usubframe, the next subframe, subframe n is a U subframe or a D subframe.If subframe n is a D subframe, as long a switching gap as a TA isinterposed between subframe [before] and subframe n. If subframe n is aU subframe, no switching gap is interposed between subframe [before] andsubframe n. Therefore, if subframe [before] is a U subframe, it shouldbe determined whether subframe n is a D subframe or a U subframe.Eventually, the following 11 cases can exist.

TABLE 8 Subframe Subframe [before] Subframe [n] [after] SET TYPE Indexin FIG. 30 D D medium set 3030 D S medium set 3035 D U short set 3050 UU D long set 3010 U D D medium set 3040 U U S long set 3015 U D S mediumset 3045 U U U medium set 3025 U D U short set 3055 S D long set 3005 SU medium set 3020

As noted from FIG. 30, the starting points of subframe sets 3005, 3010,3015, 3020, 3025 each having a U subframe as subframe n are earlier thana reference DL reception time by a TA.

If serving cells having different UL/DL configurations are active and ameasurement gap starts in subframe n of a serving cell, the followingtwo problems can occur. For the convenience of description, a servingcell that provides a reference for the start of a measurement gap isreferred to as cell CELLSTART and the other serving cells are referredto as cell CELLOTHER.

1. Overlap between a measurement gap and subframe n−1 of cell CELLOTHER.If the subframe set 3025 is cell CELLSTART and the subframe set 3030 iscell CELLOTHER, subframe n−1 of cell CELLOTHER is always a D subframeand the UE does not receive data successfully in the D subframe. Thus,if this situation occurs, the eNB should not transmit a PDSCH or anEnhanced PDCCH (EPDCCH) in subframe n−1 to the UE and the UE receives aPHICH in subframe n−1, not a PDSCH, an EPDCCH, and a Physical MulticastChannel (PMCH). The eNB can transmit a PDCCH to the UE in a PDSCH regionand this PDCCH is called an EPDCCH. When severe interference in a PDCCHregion is expected, the EPDCCH can be used and the eNB signals asubframe carrying the EPDCCH to the UE in advance. The PMCH carries aMultimedia Broadcast/Multicast Service (MBMS) signal. The PDSCH, thePMCH, and the EPDCCH are characterized by transmission across an entiresubframe in the time domain and the PHICH is transmitted only in astarting part of the subframe.

2. Overlap between a measurement gap and subframe n+6 of cell CELLOTHER.If the subframe set 3045 is cell CELLSTART and the subframe set 3055 iscell CELLOTHER, subframe n+6 is always a U subframe. If the UE performsUL transmission in subframe n+6, 6 ms is not ensured for the measurementgap.

In another embodiment of the present disclosure, the UE operates asfollows. The UE does not transmit a UL signal in subframe [after] of ashort set in cell CELLSTART. The UE does not receive a DL signal insubframe [before] and does not transmit a UL signal in subframe [after]in cell CELLOTHER. That is, the operation of the UE is summarized asfollows.

-   -   The UE determines a reference serving cell according to a preset        rule and starts a measurement gap in the reference serving cell.    -   If cell CELLSTART satisfies the following conditions, the UE        does not perform UL transmission in subframe [after]:

Subframe [before] is a D subframe (corresponding to 3050) or

Subframe [before] is a U subframe and subframe n is a D subframe(corresponding to 3055).

-   -   If subframe [before] of cell CELLOTHER is a D subframe, the UE        does not receive a PDSCH/EPDCCH/PMCH.    -   If subframe [after] of cell CELLOTHER is a U subframe, the UE        does not transmit a UL signal.

FIG. 31 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. Operations 3100, 3105, 3110, 3115, 3120, and 3125 areperformed in the same manner as operations 2900, 2905, 2910, 2915, 2920,and 2925 illustrated in FIG. 29 and thus will not be described herein.

Referring to FIG. 31, the UE determines an operation to perform insubframe [after] of a reference cell in operation 3130. If subframe[before] and subframe [after] of the reference cell are a D subframe anda U subframe, respectively, the UE does not perform UL transmission insubframe [after] of the reference cell.

If subframe [before] of the reference cell is a U subframe, a subframein which a measurement gap starts, that is, subframe n is a D subframe,and subframe [after] is a U subframe, the UE does not perform ULtransmission in subframe [after] of the reference cell.

If the condition is not satisfied, the UE determines an operation toperform in subframe [after] of the reference cell based on the type ofsubframe [after] of the reference cell.

In operation 3135, the UE determines an operation to perform in subframe[after] of a serving cell other than the reference cell.

If subframe [after] of the cell other than the reference cell is a Usubframe, the UE does not perform UL transmission in subframe [after] ofthe serving cell. If subframe [after] of the serving cell is a Dsubframe or an S subframe, the UE performs a D-subframe operation or anS-subframe operation in subframe [after] of the serving cell.

In operation 3140, the UE determines an operation to perform in subframe[before] of the serving cell other than the reference cell. If subframe[before] of the serving cell other than the reference cell is a Dsubframe, the UE receives a PHICH without a PDSCH, a PMCH, and an EPDCCHin subframe [before] of the serving cell. If subframe [before] of theserving cell is an S subframe or a U subframe, the UE performs a normaloperation in subframe [before] of the serving cell. Even though subframe[before] of the reference cell is a D subframe, the UE performs a normaloperation.

The UE performs appropriate operations in subframes [before] and [after]of serving cells as determined by the UE.

The afore-described first problem, that is, overlap between ameasurement gap and subframe [before] of a serving cell other than areference cell does not occur when the measurement gap starts in aserving cell having a last subframe starting time. For example, if ameasurement gap starts in the serving cell 3030, 3035, 3040, 3045, 3050,or 3055, the measurement gap does not overlap with subframe n−1 of aserving cell other than the reference cell.

FIG. 32 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure. Operations 3200, 3205, 3210, 3215, and 3220 are performed inthe same manner as operations 2900, 2905, 2910, 2915, and 2920illustrated in FIG. 29 and thus will not be described herein.

Referring to FIG. 32, the UE determines subframe n in which ameasurement gap will start from among subframes n of a plurality ofserving cells. The determination is made based on first and secondcriterions. The first criterion is the type of subframe n and the secondcriterion is a DL reception time.

If the types of subframes n of serving cell x and serving cell y are notsimilar, advance/delay of the starting points of subframes n of servingcell x and serving cell y is determined based on the types of thesubframes. On the other hand, if the types of subframes n of servingcell x and serving cell y are similar, advance/delay of the startingpoints of subframes n of serving cell x and serving cell y is determinedbased on DL reception times of cell x and cell y. More specifically, ifa D subframe or an S subframe coexists with a U subframe, the start ofthe U subframe precedes the start of the D subframe or the S subframe.If only U subframes or if only D or S subframes exist, advance/delaybetween the U subframes and advance/delay between the D/S subframes aredetermined according to the difference between DL reception times, thatis, the difference between propagation delays. The starting points ofsubframes having different types are spaced from each other by a TA andthe TA is longer than a propagation delay. Accordingly, whether toadvance/delay between subframes of two serving cells is first determinedaccording to the types of the subframes. If the subframes have similartypes, advance/delay between the subframes of two serving cells isdetermined according to the difference between propagation delays.

The UE determines subframe n in which a measurement gap will start fromamong subframes n of a plurality of serving cells, according to thetypes of subframes n of the serving cells. If a U subframe coexists witha D subframe or an S subframe, the UE determines to start themeasurement gap in the D subframe or the S subframe. If a plurality ofsubframes n satisfy this condition, the UE finally selects one of theplurality of subframes n based on the second criterion, that is, DLreception times. The second criterion can be the last or earliest DLreception time. If the second criterion is set as the last DL receptiontime, consistency is maintained between the first and second criteriaand thus subframe n of a serving cell having the last subframe startingtime is selected. In this case, no problem occurs if the types ofsubframes n are different. However, if the types of subframes n aresimilar, the measurement gap is overlapped with a starting part ofsubframe n+6 of a serving cell other than the reference cell. Theoverlapped part is as long as a propagation delay difference. Therefore,the UE may not perform transmission or reception. On the other hand, inthe case where the second criterion is set as the earliest DL receptiontime, if the types of subframes n are similar, the measurement gap isoverlapped with a last part of subframe n−1 of a serving cell other thanthe reference cell. The overlapped last part is as long as thepropagation delay difference. Therefore, the UE may not performtransmission or reception. From the perspective of reception, it isbetter to lose a last part of a subframe than to lose a starting part ofthe subframe. Accordingly, it may be preferred to use the earliest DLreception time as the second criterion.

After determining subframe n in which the measurement gap will start anda reference cell by the first and second criteria, the UE determines anoperation to perform in subframe [after] in operation 3230.

In operation 3230, the UE determines whether subframe n of the referencecell or the first subframe in which the measurement gap starts is a Dsubframe or a S subframe. In other words, the UE determines whether asubframe coinciding with the start of the measurement gap is a Dsubframe or an S subframe, whether subframe n of the reference cell is aU subframe, or whether subframe n−1 of the reference cell is a Dsubframe or a U subframe. If the condition is not satisfied, that is, ifsubframe n is a U subframe, this means that the start of the measurementgap is a TA earlier than if subframe n is a D subframe or an S subframeand even though subframe n+6 is a U subframe, the measurement gap is notoverlapped with subframe n+6. In operation 3235, the UE performs anormal UL transmission in subframe n+6 being a U subframe. If subframe nof the reference cell is a D subframe or an S subframe, the start of themeasurement gap coincides with the starting point of the D subframe orthe S subframe. Thus, if subframe n+6 is a U subframe, the measurementgap is overlapped with subframe n+6. Then the UE does not perform ULtransmission in subframe n+6 being a U subframe in operation 3240.

It has been described in the above embodiments of the present disclosurethat if a measurement gap starts in subframe n, the starting points ofthe measurement gap coincides with the starting boundary of subframe n.Now a description will be given of an embodiment of the presentdisclosure in which if a measurement gap starts in subframe n, thestarting points of the measurement gap coincides with the endingboundary of subframe n−1, instead of the starting boundary of subframen.

FIG. 33 illustrates an example embodiment of a setting of a measurementgap according to this disclosure.

Referring to FIG. 33, subframe n−1 (a subframe 3305) is a U subframe andsubframe n (a subframe 3315) is a D subframe. In this case, ameasurement gap starts preferably at the ending boundary of subframe n−1rather than at the starting boundary of subframe n. A gap as long as aTA 3315 is interposed between the U subframe and the D subframe and thegap 3315 is a time period during which U-to-D switching occurs. Themeasurement gap should include a time period during which a transceiverof a UE switches from a current frequency to a frequency to be measured.Accordingly, it is more efficient to directly switch from the UL to theDL of the measurement frequency than to perform U-to-D switching andthen switching to the DL of the measurement frequency. In the example ofFIG. 33, if a measurement gap 3320 starting at the starting boundary ofsubframe n is x ms long, a measurement gap 3325 starting at the endingboundary of subframe n−1 is extended by the TA 3315 and thus is (x+TA)ms long. If the measurement gap 3325 is configured to include the TA3315, an actual time period allowed as the measurement gap islengthened, from the perspective of the UE.

If subframe n+6 is a U subframe in a TDD communication system, thelength of a time period spanning from subframe n to subframe n+5 isshorter than 6 ms in most cases. Accordingly, it is preferable not toperform UL transmission in subframe n+6 being a U subframe, in terms ofcomplexity reduction of the UE. Even though UL transmission such as SRSor PUCCH transmission is scheduled in subframe n+6, the eNB ignores theUL signal, thereby preventing a problem caused by non-transmission ofthe UE.

FIG. 34 is a flowchart illustrating an example embodiment of anoperation of a UE in regards to a measurement gap according to thisdisclosure.

Referring to FIG. 34, the UE receives system information about a servingcell, acquires information required for an RRC connection establishmentprocedure, and thus starts the RRC connection setup procedure for theserving cell in operation 3400. Once the RRC connection establishmentprocedure is completed, the serving cell becomes a PCell for the UE.

In operation 3405, the UE reports its capabilities to an eNB by a UEcapability information message. If the UE supports CA, the UE capabilityinformation message includes information about UE-supported bandcombinations and the eNB determines what cell having what specificfrequency to configure as an SCell based on the supported bandcombination information.

In operation 3410, the SCell is configured for the UE. The SCell isconfigured by transmitting an RRC control message including SCellconfiguration information from the eNB to the UE and establishing asignal path for supporting the frequency band of the SCell based on theSCell configuration information by the UE, or setting a transceiver ofthe UE for data transmission and reception in the SCell. The SCellconfiguration information includes, for example, information about acenter frequency of the SCell, radio resources allocated to the UE inthe SCell, and the like.

In operation 3415, the eNB configures a measurement gap for the UE. Themeasurement gap is configured by allocating gapOffset to the UE. Forexample, gapOffset is divided into a first gapOffset ranging from 0 to39 and a second gapOffset ranging from 0 to 79 and one of the first andsecond gapOffsets is used. An MGRP corresponding to the first gapOffsetcan be 40 ms and an MGRP corresponding to the second gapOffset can be 80ms.

The SCell configuration and the measurement gap configuration can takeplace simultaneously by one RRC message or sequentially by separatecontrol messages. In the latter case, the order of the SCellconfiguration and the measurement gap configuration can be changed.

In operation 3420, the UE determines an SFN and a subframe number thatspecify the starting point of the measurement gap. For example, the SFNof a radio frame in which the measurement gap starts can be calculatedby equation (1). For example, the number of the subframe in which themeasurement gap starts can be calculated by equation (2).

In operation 3425, the UE determines the starting point of themeasurement gap by applying the SFN and the subframe number specifyingthe starting point of the measurement gap to the PCell. For example, ifsubframe number n is calculated by equation (1) and equation (2), the UEselects a subframe ending at the last time from among subframes n−1 ofcurrent active serving cells and determines to start the measurement gapat the end of the selected subframe, subframe n−1. In other words, theUE starts the measurement gap at the moment all activities are completedin subframes n−1 of the current active serving cells. The activities insubframes n−1 include a UL transmission and a DL reception scheduled insubframes n−1 and a UL transmission and a DL reception commanded insubframes n−1. In the presence of only one active serving cell, that is,in the absence of a configured and activated SCell, the UE starts ameasurement operation in the measurement gap at the end of subframe n−1of the current serving cell.

In 3430, before subframe [after] starts, the UE determines whether acurrent operation mode is an FDD mode or a TDD mode. In the FDD mode,the UE goes to operation 3435 and in the TDD mode, the UE goes tooperation 3440.

In operation 3435, the UE omits a UL transmission scheduled in subframe[after]. The UE determines whether subframe [after] is a D subframe oran S subframe in operation 3440. If subframe [after] is a D subframe oran S subframe, the UE performs an appropriate operation according to thetype of subframe [after] in operation 3445. That is, if subframe [after]is a D subframe, the UE receives a DL signal on a PHICH/PDCCH/PDSCH andif subframe [after] is an S subframe, the UE receives a DL signal on aPHICH/PDCCH and then switches to a UL transmission mode.

If subframe [after] is neither a D subframe nor an S subframe, that is,if subframe [after] is a U subframe, the UE does not perform anoperation scheduled in subframe [after] in operation 3450.

While various embodiments of the present disclosure regarding setting ofa measurement gap have been described above, it is to be clearlyunderstood that the present disclosure is not limited to theafore-described specific operations. That is, at least a part of theoperations according to the foregoing embodiments of the presentdisclosure can be combined or omitted within the scope of the presentdisclosure.

Another embodiment of the present disclosure provides Rank Indication(RI) reporting.

A UE reports, to an eNB, information indicating the number of layersthat the UE can receive according to the channel state of a serving cellfor which a MIMO operation has been configured. This information iscalled an RI.

An RI can be set on a serving cell basis. Preferably, the size of the RIis set according to a maximum number of layers that can be transmittedand received theoretically in the serving cell. For example, if up tofour layers can be transmitted and received, the RI is preferably 2bits. If up to eight layers can be transmitted and received, the RI ispreferably 3 bits.

The maximum number of layers transmittable and receivable in a servingcell is determined to be the smaller between a maximum number of layersreceivable at the UE (such as the MIMO capability of the UE) and amaximum number of layers transmittable from the eNB (such as the numberof antenna ports configured at the eNB). The maximum number of layerssupported by a UE on a DL is indicated by the UE category of the UE. AUE category is an index indicating a combination of parameters thatdefine UL and DL capabilities of a UE. For example, the following eightcategories can be defined as listed in TABLE 9.

TABLE 9 Maximum number of DL-SCH Maximum Maximum transport block bitsnumber of bits of number of received within a a DL-SCH supported layersTTI (In carrier transport block for spatial aggregation received withina Total number of multiplexing in UE Category operation) TTI softchannel bits DL Category 1 10296 10296 250368 1 Category 2 51024 510241237248 2 Category 3 102048 75376 1237248 2 Category 4 150752 753761827072 2 Category 5 299552 149776  3667200 4 Category 6 301504 149776 3654144 2 or 4 (4 layers) 75376 (2 layers) Category 7 301504 149776 3654144 2 or 4 (4 layers) 75376 (2 layers) Category 8 2998560 299856 35982720 8

In TABLE 9, ‘Maximum number of supported layers for spatial multiplexingin DL’ is information related to MIMO capabilities of a UE.

Category 1 to Category 5 are defined in LTE Release 8 and Release 9,whereas Category 6, Category 7, and Category 8 are defined in LTERelease 10. Therefore, an eNB conforming to LTE Release 8 or Release 9does not understand Category 6, Category 7, and Category 8 reported byUEs and UEs do not recognize an LTE release to which the eNB conforms.Therefore, a UE under Category 6, Category 7, and Category 8 reports oneof Category 6, Category 7, and Category 8 and one of Category 1 toCategory 5 to the eNB. For the convenience of description, a MIMOcapability related to Category 1 to Category 5 is referred to as a firstMIMO capability and a MIMO capability related to Category 6, Category 7,and Category 8 is referred to as a second MIMO capability.

For a MIMO capability mapped to a UE category, only one value can bereported for all frequency bands. However, the number of layers that aUE can receive can vary with frequency bands. For example, while the UEcan receive as many layers as the number of antennas in a high frequencyband, the UE can receive a smaller number of layers in a low frequencyband because the distance between antennas is decreased to below a halfof a frequency wavelength. The UE reports a MEMO capability on a bandbasis or on a band combination basis, thereby preventing the problemcaused by different frequency bands. A MIMO capability for each bandcombination is reported in a supported MIMO-Capability InformationElement (IF) of a supportedBandCombination IE. supportedBandCombinationwas introduced to Release 10. A MIMO capability reported on a bandcombination basis is referred to as a third MIMO capability.

The UE reports its MIMO capability to the eNB as one, two, or threetypes of MIMO capabilities. A UE under Category 6, Category 7, orCategory 8 reports the first, second, and third MIMO capabilities, a UEconforming to Release 10 or a higher-version Release under Category 1 toCategory 5 reports the first and third MIMO capabilities, and a UEconforming to Release 8 or Release 9 reports the first MIMO capability.

The bit width (that is the number of bits) to be used in reporting an RIis determined based on a MIMO capability reported by a UE and specificconfiguration information indicated to the UE by an eNB. A DLtransmission mode can further be considered. The UE is configured toreceive a PDSCH in one of a plurality of transmission modes by higherlayer signaling. A transmission mode defines the format of controlinformation (such as a PDCCH) related to a PDSCH, a resource area to besearched for control information (a search space), and a transmissionscheme of the PDSCH. The transmission scheme of the PDSCH includes, forexample, an antenna configuration, transmit diversity, spatialmultiplexing, Multi User (MU) MIMO, and the like.

Transmission modes, TM1 to TM8 are defined in LTE Release 8 and Release9 and transmission modes TM9 and TM10 are defined in LTE Release 10 or ahigher-version Release. Accordingly, if the eNB configures TM9 or TM10,the UE can determine that the eNB conforms to LTE Release 10 or ahigher-version Release. Since the eNB conforming to LTE Release 10 or ahigher-version Release may understand a MIMO capability for each bandcombination, the bit width for an RI can be determined based on the MIMOcapability for each band combination.

FIG. 35 is a diagram illustrating a signal flow for an overall operationfor determining the bit width for an RI according to an embodiment ofthe present disclosure.

Referring to FIG. 35, a UE 3505 reports MIMO capability information toan eNB 3510 in operation 3513. The MIMO capability information canindicate a part or all of the first, second, and third MIMO capabilitiesaccording to the category and Release of the UE.

The eNB 3510 determines based on a MIMO capability of the UE 3505 and aMIMO capability of the eNB 3510 whether to configure MIMO for the UE3505 and how many antennas to configure for a MIMO operation of the UE3505 and generates MIMO configuration information based on thedetermination in operation 3515.

The eNB 3510 transmits the MIMO configuration information, for example,information indicating the number of antenna ports to be used andinformation indicating whether an RI is to be reported to the UE 3505 bya predetermined control message in operation 3520. The UE 3505 and theeNB 3510 determine the bit width for an RI according to predeterminedinformation and a predetermined rule in operation 3525. Because the UEand the eNB use the same algorithm, they acquire the same bit width forthe RI. The predetermined information and the predetermined rule will bedescribed later. In operation 3530, the UE 3505 reports an RI in thedetermined bit width and the eNB 3510 determines the number of layers tobe transmitted to the UE 3505 by receiving and interpreting the RIaccording to the bit width for the RI.

FIG. 36 is a flowchart illustrating an example embodiment of anoperation of a UE for determining the bit width for an RI according toan embodiment of the present disclosure. While the following descriptionis given of a UE operation, an eNB determines the bit width for an RI bythe same algorithm.

Referring to FIG. 36, a UE receives system information about a servingcell, acquires information required to establish an RRC connectionestablishment procedure from the system information, and starts the RRCconnection establishment procedure for the serving cell in operation3600. Upon completion of the RRC connection establishment procedure, theserving cell becomes a PCell for the UE.

In operation 3605, the UE reports its capability to the eNB by acapability information message. If the UE supports CA, the UE includesinformation about UE-supported band combinations in the capabilityinformation message and the eNB determines what cell having whatspecific frequency to configure as an SCell for the UE based on thesupported band combination information. The capability informationmessage includes the following MIMO information.

-   -   If the UE conforms to Release 8 or Release 9, first MIMO        capability.    -   If the UE conforms to Release 10 or a higher-version Release and        belongs to Category 1 to Category 5, the first MIMO capability        and third MIMO capability.    -   If the UE conforms to Release 10 or a higher-version Release and        belongs to Category 6 to Category 8, the first MIMO capability,        a second MIMO capability, and the third MIMO capability.

In operation 3610, MIMO is configured for the serving cells of the UE.Then the UE receives a control message indicating reporting of an RI forthe serving cell from the eNB.

In operation 3615, the UE determines the bit width for the RI to bereported for the serving cell based on predetermined informationaccording to a predetermined rule. Herein, the UE can use one of thefollowing rules.

[Rule 1]

-   -   A reference UE capability is determined based on a transmission        mode configured for a serving cell. That is, if the transmission        mode is Transmission Mode (TM) 9 or TM10, the third MIMO        capability is determined to be the reference UE capability.        Otherwise, for example, if the transmission mode is TM4 or TM8,        the first MIMO capability determined to be the reference UE        capability.    -   An eNB reference capability is determined based on the antenna        configuration of the eNB. That is, the antenna configuration of        the eNB can include a maximum number of antenna ports available        for data transmission and can be indicated to the UE by system        information or a dedicated RRC message.    -   The bit width for an RI is determined based on the minimum of        the number of layers corresponding to the UE reference        capability and the number of layers corresponding to the eNB        reference capability. If the minimum value is 4, the RI is 2        bits and if the minimum value is 8, the RI is 3 bits.

If TM4 or TM8 is configured by the eNB, this means that the eNB ishighly likely to be a Release 8 eNB or a Release 9 eNB. If TM9 or TM10is configured by the eNB, this means that the eNB is highly likely to bea Release 10 eNB or an eNB conforming to a higher-version Release. TheRelease 10 eNB or an eNB conforming to a higher-version Release canunderstand the third MIMO capability.

The third MIMO capability for a serving cell means a MIMO capabilityreported in supportedBandCombination corresponding to a configurationfor a corresponding time point. For example, if one serving cell isconfigured in band X and one serving cell is configured in band Y, athird MIMO capability for the serving cell of band X is a MIMOcapability reported for band X in supportedBandCombination configuredfor band X and band Y. In other words, considering the third MIMOcapability, the number of layers corresponding to a MIMO capabilityreported for the frequency band of a serving cell is determined to theUE reference capability.

[Rule 2]

-   -   If at least two serving cells are configured, a third MIMO        capability for a corresponding serving cell is determined to be        a UE reference capability.    -   If only one serving cell is configured and TM9 or TM10 is        configured for a UE, a third MIMO capability for the serving        cell is determined to be a UE reference capability.    -   If only one serving cell is configured and TM4 or TM8 is        configured for a UE, a first MIMO capability is determined to be        a UE reference capability.    -   An eNB reference capability is determined based on the antenna        configuration of an eNB.    -   The bit width for an RI is determined according to the minimum        of the number of layers corresponding to the UE reference        capability and the number of layers corresponding to the eNB        reference capability.

[Rule 3]

-   -   A UE reference capability is determined according to a        transmission mode configured for a serving cell. If TM4 or TM8        is configured in the serving cell for a UE, a first MIMO        capability is determined to be a UE reference capability. If TM9        or TM10 is configured in the serving cell and the UE belongs to        Category 1 to Category 5, the first MIMO capability is        determined to be the UE reference capability. If TM9 or TM10 is        configured and the UE belongs to Category 6 to Category 8, a        second MIMO capability is determined to be the UE reference        capability.    -   An eNB reference capability is determined based on the antenna        configuration of an eNB.    -   The bit width for an RI is determined according to the minimum        of the number of layers corresponding to the UE reference        capability and the number of layers corresponding to the eNB        reference capability.

As noted from TABLE 9, the second MIMO capability for Category 6 andCategory 7 is represented by two values, 2 and 4. This is becausedifferent MIMO capabilities of a UE for CA and non-CA are considered. Ifthe second MIMO capability is used as a UE reference capability for a UEunder Category 6 or Category 7, the value of the second MIMO capabilityis determined according to the following rule.

If only one serving cell is configured, the second MIMO capability isthe higher between the two values, that is, 4.

If two serving cells are configured, the second MIMO capability 2 is thelower between the two values, that is, 2.

[Rule 4]

-   -   If a predetermined indicator is received in a predetermined        control message from an eNB, a third MIMO capability for a        corresponding serving cell is determined to be a UE reference        capability. The indicator can be used to command a UE to use the        third MIMO capability.    -   If the predetermined indicator is not received in the        predetermined control message from the eNB, a first MIMO        capability is determined to be the UE reference capability.    -   An eNB reference capability is determined based on the antenna        configuration of an eNB.    -   The bit width for an RI is determined according to the minimum        of the number of layers corresponding to the UE reference        capability and the number of layers corresponding to the eNB        reference capability.

The eNB does not transmit the indicator to a Release 8 UE or a Release 9UE. Only an eNB that can understand the third MIMO capability transmitsthe predetermined indicator. Therefore, the UE determines what MIMOcapability to be used as the UE reference capability depending on thepresence or absence of the indicator.

The indicator can be transmitted only in a control message. The controlmessage can be, for example, an RRC control message includinginformation indicating RI reporting or MIMO configuration information.

In operation 3620, the UE encodes an RI based on the determined bitwidth for the RI and transmits the coded RI in predeterminedtransmission resources at a predetermined time point to the eNB.

After establishing an RRC connection for the UE, receiving thecapability information message, and indicating RI reporting to the UE,the eNB determines the bit width for the RI according to the same ruleand information as used in the UE in operation 3615. The eNB can receivethe RI having the determined bit width from the UE in predeterminedtransmission resources at a predetermined time from the UE and candecode the RI normally.

The UE and the eNB that perform the operations illustrated in FIGS. 30to 36 have the configurations illustrated in FIGS. 11 and 12,respectively.

The proposed method and apparatus for performing communication using TDDcells having different frequency bands can be implemented ascomputer-readable code in a computer-readable recording medium. Thecomputer-readable recording medium can include any kind of recordingdevice storing computer-readable data. Examples of the recording mediumcan include Read Only Memory (ROM), Random Access Memory (RAM), opticaldisk, magnetic tape, floppy disk, hard disk, non-volatile memory, andthe like, and can also include the medium that is implemented in theform of carrier waves (for example, transmission over the Internet). Inaddition, the computer-readable recording medium can be distributed overthe computer systems connected over the network, and computer-readablecodes can be stored and executed in a distributed manner.

Although the present disclosure has been described with an exemplaryembodiment, various changes and modifications may be suggested to oneskilled in the art. It is intended that the present disclosure encompasssuch changes and modifications as fall within the scope of the appendedclaims.

What is claimed is:
 1. A method for performing communication in a basestation (BS) of a wireless communication system, the method comprising:transmitting, to a user equipment (UE), information about an antennaconfiguration of the BS; transmitting, to the UE, control informationindicating a transmission mode for a serving cell; and receiving, fromthe UE, information about a rank indication (RI) based on a number ofbits for the RI, wherein the number of bits for the RI is determinedbased on a Multiple Input Multiple Output (MIMO) capability reported tothe BS by the UE, if the identified transmission mode is a firsttransmission mode, and wherein the number of bits for the RI isdetermined based on the antenna configuration, if the identifiedtransmission mode is a second transmission mode.
 2. The method of claim1, wherein the first transmission mode includes at least one of atransmission mode 9 and a transmission mode 10 configured to the UE. 3.The method of claim 1, wherein, if the identified transmission mode isthe second transmission mode, the number of bits for the RI isdetermined by: identifying a MIMO capability reported to the BS by theUE; determining the number of layers corresponding to the MIMOcapability; determining the number of antenna ports based on the antennaconfiguration of the BS; and determining the number of bits for the RIbased on the smaller value between the number of layers corresponding tothe MIMO capability and the determined number of antenna ports.
 4. Themethod of claim 1, wherein, if the identified transmission mode is thesecond transmission mode, the number of bits for the RI is determinedbased on a UE category of the UE and the antenna configuration.
 5. Themethod of claim 1, wherein the information about the antennaconfiguration of the BS is transmitted to the UE via a Radio ResourceControl (RRC) message.
 6. An apparatus of a base station (BS) forperforming communication in a wireless communication system, theapparatus comprising: a transmitter configured to transmit, to a userequipment (UE), information about an antenna configuration of the BS andcontrol information indicating a transmission mode for a serving cell;and a receiver configured to receive, from the UE, information about arank indication (RI) based on a number of bits for the RI, wherein thenumber of bits for the RI is determined based on a Multiple InputMultiple Output (MIMO) capability reported to the BS by the UE, if theidentified transmission mode is a first transmission mode, and whereinthe number of bits for the RI is determined based on the antennaconfiguration, if the identified transmission mode is a secondtransmission mode.
 7. The apparatus of claim 6, wherein the firsttransmission mode includes at least one of a transmission mode 9 and atransmission mode 10 configured to the UE.
 8. The apparatus of claim 6,wherein the number of bits for the RI is determined by: identifying aMEMO capability reported to the BS by the UE, if the identifiedtransmission mode is the second transmission mode; determining thenumber of layers corresponding to the MEMO capability; determining thenumber of antenna ports based on the antenna configuration of the BS;and determining the number of bits for the RI based on the smaller valuebetween the number of layers corresponding to the MIMO capability andthe determined number of antenna ports.
 9. The apparatus of claim 6,wherein, if the identified transmission mode is the second transmissionmode, the number of bits for the RI is determined based on a UE categoryof the UE and the antenna configuration.
 10. The apparatus of claim 6,wherein the information about the antenna configuration of the BS istransmitted to the UE via a Radio Resource Control (RRC) message.
 11. Amethod for performing communication in a user equipment (UE) of awireless communication system, the method comprising: receiving, from abase station (BS), information about an antenna configuration of the BS;receiving, from the BS, control information indicating a transmissionmode for a serving cell; and transmitting, to the BS, information abouta rank indication (RI) based on a number of bits for the RI, wherein thenumber of bits for the RI is determined based on a Multiple InputMultiple Output (MIMO) capability reported to the BS by the UE, if theidentified transmission mode is a first transmission mode, and whereinthe number of bits for the RI is determined based on the antennaconfiguration, if the identified transmission mode is a secondtransmission mode.
 12. The method of claim 11, wherein the firsttransmission mode includes at least one of a transmission mode 9 and atransmission mode 10 configured to the UE.
 13. The method of claim 11,wherein, if the identified transmission mode is the second transmissionmode, the number of bits for the RI is determined by; identifying a MIMOcapability reported to the BS by the UE; determining the number oflayers corresponding to the MIMO capability; determining the number ofantenna ports based on the antenna configuration of the BS; anddetermining the number of bits for the RI based on the smaller valuebetween the number of layers corresponding to the MIMO capability andthe determined number of antenna ports.
 14. The method of claim 11,wherein, if the identified transmission mode is the second transmissionmode, the number of bits for the RI is determined based on a UE categoryof the UE and the antenna configuration.
 15. The method of claim 11,wherein the information about the antenna configuration of the BS istransmitted to the UE via a Radio Resource Control (RRC) message.
 16. Anapparatus of a user equipment (UE) for performing communication in awireless communication system, the apparatus comprising: a receiverconfigured to receive, from a base station (BS), information about anantenna configuration of the BS and control information indicating atransmission mode for a serving cell; and a transmitter configured totransmit, to the BS, information about a rank indication (RI) based on anumber of bits for the RI, wherein the number of bits for the RI isdetermined based on a Multiple Input Multiple Output (MIMO) capabilityreported to the BS by the UE, if the identified transmission mode is afirst transmission mode, and wherein the number of bits for the RI isdetermined based on the antenna configuration, if the identifiedtransmission mode is a second transmission mode.
 17. The apparatus ofclaim 16, wherein the first transmission mode includes at least one of atransmission mode 9 and a transmission mode 10 configured to the UE. 18.The apparatus of claim 16, wherein the number of bits for the RI isdetermined by: identifying a MIMO capability reported to the BS by theUE, if the identified transmission mode is the second transmission mode;determining the number of layers corresponding to the MIMO capability;determining the number of antenna ports based on the antennaconfiguration of the BS; and determining the number of bits for the RIbased on the smaller value between the number of layers corresponding tothe MIMO capability and the determined number of antenna ports.
 19. Theapparatus of claim 16, wherein, if the identified transmission mode isthe second transmission mode, the number of bits for the RI isdetermined based on a UE category of the UE and the antennaconfiguration.
 20. The apparatus of claim 16, wherein the informationabout the antenna configuration of the BS is transmitted to the UE via aRadio Resource Control (RRC) message.